Create doc on build arg best practices for Blimp.

Currently this information is either not captured in docs or widely spread.

This document provides the key guidelines and best practices for creating and using a new Chrome build arg.

BUG=593073

Review URL: https://codereview.chromium.org/1772213002

Cr-Commit-Position: refs/heads/master@{#380702}

1 files changed, 26 insertions, 3 deletions

diff --git a/blimp/docs/build.md b/blimp/docs/build.md
index a235121..e5e20d7 100644
--- a/blimp/docs/build.md
+++ b/blimp/docs/build.md
@@ -3,10 +3,12 @@ Blimp only supports building using [GN](../../tools/gn/README.md). A quick
 overview over how to use GN can be found in the GN
 [quick start guide](../../tools/gn/docs/quick_start.md).
 
-There are three different build configurations depending on what you want to
+## Building
+
+There are two different build configurations depending on what you want to
 build:
 
-## Android client
+### Android client
 
 Create an out-directory and set the GN args:
 
@@ -35,7 +37,7 @@ To add your own build preferences
 gn args out-android/Debug
 ```
 
-## Engine
+### Engine
 
 Create another out-directory and set the GN args:
 
@@ -56,3 +58,24 @@ To add your own build preferences
 ```bash
 gn args out-android/Debug
 ```
+
+## Adding new build arguments
+
+Adding new build arguments should be fairly rare. Arguments first need to be
+[declared](../../tools/gn/docs/quick_start.md#Add-a-new-build-argument).
+
+They can then be used to change how the binary is built or passed through to
+code as a
+[defines](../../tools/gn/docs/reference.md#defines_C-preprocessor-defines).
+
+Finally the Blimp argument templates should be updated to reflect the
+(non-default for Chrome) behavior desired by Blimp (see below).
+
+## Updating bulid arguments in templates
+
+Build argument templates exist for the client and engine at
+[`build/args/blimp_client.gn`](../../build/args/blimp_client.gn) and
+[`build/args/blimp_engine.gn`](../../build/args/blimp_engine.gn).
+
+These can be updated as in the same manner as your personal `args.gn` files
+to override default argument values.