add a basic CONTRIBUTING.md (#535)

Fixes: #359

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+First off, thank you for taking an interest to contribute to this project!
+
+## Opening issues
+
+When opening a [new issue](https://github.com/golang/mock/issues/new/choose)
+please:
+
+1. Make sure there are not other open/closed issues asking/reporting/requesting
+   the same thing.
+1. Choose one of our provided templates and fill out as much information as
+   possible.
+
+## Opening a Pull Requests
+
+We gladly accept contributions from the community. Before opening a pull request
+please make sure to create an issue for discussion first. This helps us decide
+what action should be taken in regards to the issue.