Update readme with usage information

git-svn-id: https://plugins.svn.wordpress.org/instamojo/trunk@886741 b8457f37-d9ea-0310-8a92-e5e31aec5664
Fix readme

git-svn-id: https://plugins.svn.wordpress.org/instamojo/trunk@886742 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add contribution guidelines

git-svn-id: https://plugins.svn.wordpress.org/instamojo/trunk@886753 b8457f37-d9ea-0310-8a92-e5e31aec5664