GitHub friendly version of readme.

git-svn-id: https://plugins.svn.wordpress.org/simple-ldap-login/trunk@768004 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme.md

git-svn-id: https://plugins.svn.wordpress.org/simple-ldap-login/trunk@768005 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add contributing notes to readme. Revise get help instructions.

git-svn-id: https://plugins.svn.wordpress.org/simple-ldap-login/trunk@768006 b8457f37-d9ea-0310-8a92-e5e31aec5664