Add contribution guide

CONTRIBUTING.md

+How to contribute to the ELPA library:
+
+We are very happy and gratefull if you are willing to help us improve ELPA. Thus, we would like to make this process as simple as possible for you, 
+but at the same time still keep it manageable for us
+
+For recommendations and suggestions, a simple email to us is sufficient!
+
+If you would like to share with us your improvements, we suggest the following ways:
+
+1. If you use a public accessible git repository, please send us a merge request. This is the preferred way
+2. An email with a patch, will also be ok.
+
+Thank you for supporting ELPA!
+
+The ELPA development team