Add to README

add contribution documentation.

Author: Dean Karn

README.md

@@ -22,3 +22,15 @@ Usage
 =====
 
 Please see http://godoc.org/gopkg.in/joeybloggs/go-validate-yourself.v1 for detailed usage docs.
+
+Contributing
+============
+
+There will be a development brach for each version of this package i.e. v1-development, please
+make your pull requests against this branch.
+
+If changes are breaking please create an issue, I'll create a new version that you can make a pull
+request against, and then review from there.
+
+I strongly encourage everyone whom creates a custom validation function to contribute them and
+help make this package even better.