[Docs] Add information on how to contribute to the README

I noticed after reading a few GitHub comments that the information
about contributing to V8 is hard to find. I think it makes sense
to add this information to the README.

NOTRY=true
R=jochen@chromium.org, seththompson@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1342803004

Cr-Commit-Position: refs/heads/master@{#30730}

README.md

@@ -31,3 +31,10 @@ configuration in `.git/config`:
 
         fetch = +refs/branch-heads/*:refs/remotes/branch-heads/*
         fetch = +refs/tags/*:refs/tags/*
+
+
+Contributing
+=============
+
+Please follow the instructions mentioned on the 
+[V8 wiki](https://code.google.com/p/v8-wiki/wiki/Contributing).