Add contribution guidelines

CONTRIBUTNG.md

@@ -0,0 +1,16 @@
+# Contributing
+
+Open-Source means we welcome all changes and ideas any time of day and nights, through issues and pull requests.
+
+## How to contribute
+
+As the community around this project is small enough, we set the following contributing guidelines:
+ - if necessary, [open an issue](https://github.com/pandatix/go-cvss/issues/new) first
+ - fork the repository
+ - clone, commit, and when feeling confident enough [create a pull request](https://github.com/pandatix/go-cvss/compare) with the issue(s) it solves.
+ - check your notifications for comments, suggestions, and maybe an approval
+
+## Quality
+
+This implementation focuses on the first.org specifications, security and performances.
+We expect your contribution to provide a minimum quality with unitary tests.