completed readme for now.

.gitignore

@@ -1,3 +1,4 @@
 /target
 ngen.toml
 build.ninja
+example/

README.md

@@ -41,6 +41,8 @@ Uninstall: `sudo sh uninstall.sh`
 What follows is a tutorial of how to set up ngen for an existing executable
 project. If you are looking for a reference, look <SOMEWHERE>.
 
+### Basics
+
 The first thing you will need is an `ngen.toml` file. This is what will specify
 all of the build parameters, such as compilation flags, source files to track,
 etc. Start by creating this file and opening it in your editor.
@@ -168,6 +170,122 @@ these values will be automatically picked up by Ninja and accounted for in the
 build. Running `ninja -v` immediately after saving `ngen.toml` should show that
 the options you set were recognized, and your files were rebuilt accordingly.
 
-TODO: explain
-- seperate targets
-- config table
+Now, this is a good start. But, it often the case that in a project, you want to
+have multiple different build *targets,* or configurations, that build the
+project with slightly different parameters; for example, it is common to have a
+"debug" build target that is unoptimized and includes debugging symbols, and a
+"release" build that is optimized at compile time. ngen is designed to make this
+configuration as easy as possible.
+
+### Targets
+
+When we were specifying parameters above, we were doing so in the "global
+scope," so to speak, of the TOML file. Without knowing it, we were actually
+configuring the `main` target (this is why the outfiles were placed in
+`build/*main/*`. The main target is special because it does not need to be
+labeled: all build parameters placed in the global scope (or, more accurately,
+the TOML"root table"), will be used for the `main` target. To specify
+additional targets, we create a TOML table with the name of our target. Let's
+create a target called `debug`. Add the following to your `ngen.toml`:
+
+```toml
+[debug]
+outfile = "example_dbg"
+compiler_flags = ["-g"]
+```
+
+What's going on here? How does `debug` know what files to operate on, what
+compiler to use, etc? Well, the `main` target is special in another way: all
+targets *inherit* the parameters set in the main target. Inheritance works
+according to two simple rules: **a**rrays **a**ppend, **s**trings **s**upercede.
+
+The first thing that happens is `debug` takes on all the same parameters from
+main. Then, ngen reads the outfile key in `debug`. Becuase outfile is a string,
+`debug.outfile` is overwritten as the value specified, in this case
+"example\_dbg". On the othe hand, since compiler\_flags is a list, the elements
+sepecified in `debug.compiler_flags` are appended to the list of flags specified
+in `main`. So in this case, the effective value of `debug.compiler_flags` is
+`["-Wall", "-Wextra -O2", "-g"]`. 
+
+For our debug build, we probably don't want the `-O2` flag---optimizations
+should only happen in a "release" type build. So, lets remove the `-O2` flag
+from `main` and place it in a new target called `release`. Our `ngen.toml`
+should now look like this:
+
+```toml
+outfile = "example"
+compiler = "gcc"
+compiler_flags = ["-Wall", "-Wextra"]
+linker_libs = ["-lm"]
+sources = [
+  "src/main.c",
+  "src/util.c",
+  "src/functions.c",
+  "src/foobar.c",
+]
+
+[debug]
+outfile = "example_dbg"
+compiler_flags = ["-g"]
+
+[release]
+compiler_flags = ["-O2"]
+```
+
+This brings up an important design pattern you should keep in mind when writing
+your `ngen.toml`: the special `main` target should only contain the largest
+subset of all your build parameters. Additonal targets should add specific
+parameters for specific use cases, as we saw in the above example.
+
+In a nutshell, inheritance allows you to easily create multiple targets with small variations,
+without having to rewrite the same thing over and over again.
+
+Save `ngen.toml`, and try running `ninja -v debug` or `ninja -v release`. You
+should see that each of these targets uses the parameters that we specified with
+inheritance. Outfiles for a given target are always placed in
+`build/<target_name>/`, as you can see with `tree build`. This keeps things
+organized, and also means we don't have to specify a different outfile name for
+each target (I did above just to show you how strings are replaced in the
+inheritance system).
+
+Note that by default, running `ninja` alone with no target specifed will run
+*every single target* it finds. You can change this behavor by adding the
+`default = true` key to the targets you want to be built when Ninja is invoked
+with no arguments. Say that this example project is under active developemnt,
+and you will be building the `debug` target alot. You can add the `default =
+true` flag to the `[debug]` table, and now running `ninja` by it self will only
+build the `debug` target. You can still build the release and main targets by
+running `ninja release` and `ninja main`.
+
+It is easy to see how powerful this simple configuration file is already.
+However, ngen has a few more features that you may find useful.
+
+### Configuration
+
+The `config` table is where you can specify certain options which change the way
+ngen behaves. Right now, this only involves a single feature: generating a
+compile\_commands.json file for the `clangd` LSP.
+
+To enable the generation of compile\_commands.json, simply add the following
+line the **global scope** (i.e. next to your `main` target, not beneath any of
+your other targets) of your `ngen.toml`:
+
+```toml
+config.compile_commands = true
+```
+
+The next time you run Ninja, ngen will automatically generate
+`build/compile_commands.json` where it can be picked up by clangd. Ninja will
+make sure that this file is kept up to date as well. Just set the option in your
+ngen.toml and forget about it.
+
+By default, the compile\_commands.json will be generated according to the build
+specs of the `main` target. To change which target it is generated for use the
+`config.compile_commands_target` key. For example,
+
+```toml
+config.compile_commands_target = "debug"
+```
+
+will generate the the compile\_commands for the `debug` target.
+