Update Build and Running documentation.

blimp/docs/build.md

Index: blimp/docs/build.md
diff --git a/blimp/docs/build.md b/blimp/docs/build.md
index 638cefa76f78b2e30f1f7ded48905db4311de2d5..0df09d756a38cb60926cdaa8cd3c50a7a1ad48fd 100644
--- a/blimp/docs/build.md
+++ b/blimp/docs/build.md
@@ -1,3 +1,10 @@
+[TOC]
+
+# Checkout
+If you want to build the Android client then you will need to follow
+instructions [here](https://www.chromium.org/developers/how-tos/android-build-instructions)
+to sync Android related code as well.
+
 # Using GN
 Blimp only supports building using [GN](../../tools/gn/README.md). A quick
 overview over how to use GN can be found in the GN
@@ -45,6 +52,10 @@ To add your own build preferences:
 gn args out-android/Debug
 ```
 
+For example, you can build `x86` APK by adding `target_cpu = "x86"` to the `gn
+args`.
+
+
 ### Engine
 
 Create another out-directory and set the GN args: