Add Pull Request Instructions

Standardize the way pull requests are done.  

The guide covers:
* one feature one commit
* tox before pushing
* branching off `upstream/develop`

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,7 @@
+**Please See** [Git Pull Request Instructions](https://github.com/aws/aws-parallelcluster/wiki/Git-Pull-Request-Instructions)
+
 *Issue #, if available:*
 
 *Description of changes:*
 
-
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.