Create doc on build arg best practices for Blimp.

tools/gn/docs/quick_start.md

 # GN Quick Start guide
 
 [TOC]
 
 ## Running GN
 
 You just run `gn` from the command line. There is a script in
 depot\_tools (which is presumably on your path) with this name. The
 script will find the binary in the source tree containing the current
 directory and run it.
@@ skipping 54 matching lines @@
 
 target_cpu = "arm"
 target_cpu = "x86"
 target_cpu = "x64"
 ```
 
 See [GNCrossCompiles](cross_compiles.md) for more info.
 
 ## Configuring goma
 
-
 Run `gn args out/Default` (substituting your build directory as needed).
 Add:
 
 ```
 use_goma = true
 goma_dir = "~/foo/bar/goma"
 ```
 
 If your goma is in the default location (`~/goma`) then you can omit the
 `goma_dir` line.
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 ```
 > ninja -C out/Default say_hello
 ninja: Entering directory 'out/Default'
 [1/1] Regenerating ninja files
 [4/4] LINK say_hello
 > out/Default/say_hello
 Hello, Bill and Joy.
 ```
 
+## Add a new build argument
+
+You declare which arguments you accept and specify default values via
+`declare_args`.
+
+```
+declare_args() {
+  enable_teleporter = true
+  enable_doom_melon = false
+}
+```
+
+See `gn help buildargs` for an overview of how this works.
+See `gn help declare_args` for specifics on declaring them.
+
+It is an error to declare a given argument more than once in a given scope, so
+care should be used in scoping and naming arguments.
+
 ## Don't know what's going on?
 
 You can run GN in verbose mode to see lots of messages about what it's
 doing. Use `-v` for this.
 
 ### Print debugging
 
 There is a `print` command which just writes to stdout:
 
 ```
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 line flag. This will output a summary of timings for various things.
 
 You can also make a trace of how the build files were executed:
 
 ```
 gn --tracelog=mylog.trace
 ```
 
 and you can load the resulting file in Chrome's `about:tracing` page to
 look at everything.