Create doc on build arg best practices for Blimp.

blimp/docs/build.md

Index: blimp/docs/build.md
diff --git a/blimp/docs/build.md b/blimp/docs/build.md
index a235121d4852a023b5e6970b3ba408e8d4089a04..7ce13866ce2f422b91876af5ce7732b809f6d5b0 100644
--- a/blimp/docs/build.md
+++ b/blimp/docs/build.md
@@ -2,8 +2,10 @@
 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).
+If you want to add new arguments, see the
+[build arguments guide](args.md).

[Dirk Pranke, 2016/03/09 01:28:45]

I would try to distinguish between creating an entirely new flag (which we
should be trying to avoid, but if we do need to do it, try to document it
generically in the GN docs) and needing to set a particular flag to a particular
value (which we do need), in which case you need to update the build template
files in that case ...

[Jess, 2016/03/09 19:14:27]

Done.

 
-There are three different build configurations depending on what you want to
+There are two different build configurations depending on what you want to
 build:
 
 ## Android client