Expand README to explain directory structure and how this repo is used.

BUG=skia:
R=borenet@google.com

Review URL: https://codereview.chromium.org/344213002

README

diff --git a/README b/README
index bddda1e..c6bc70c 100644
--- a/README
+++ b/README
@@ -1,2 +1,13 @@
 This repository contains code shared by the skia and skia-buildbot repositories.
 
+The file layout is done by language, such as the following directories
+for Python, Go and Javascript libraries.
+
+  /py
+  /go
+  /js
+
+This repository is pulled into both https://skia.googlesource.com/skia/
+and https://skia.googlesource.com/buildbot as /common in both repositories
+so that references to common files should be the same regardless
+of which repository the code is in.