Adding contribution guide

Author: sjlu

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+## Version bumps
+
+As much as I love version bump pull requests, they really do not provide any value
+since I have to make a release manually anyways. And lets be honest, its a matter
+of just changing a version number. So please, instead of making these as pull
+requests, just ask through an [issue](https://github.com/sjlu/CodeIgniter-Bootstrap/issues).
+
+## Worthwhile contributions
+
+* Things that are explicitly broken and fixed are always welcome
+* Code that is worthwile to everyone in [files](https://github.com/sjlu/CodeIgniter-Bootstrap/tree/master/files)