Add contributing guidelines (#227)

It provides some basic information, but mostly links to the main
repo's guidelines to reduce duplication and make maintaining the
guidelines easier.

.github/CONTRIBUTING.md

@@ -0,0 +1,7 @@
+# Contributing to POP
+
+First, thank you so much for wanting to contribute! It means so much that you care enough to want to contribute. We appreciate every PR from the smallest of typos to the be biggest of features.
+
+Please open PRs against the `development` branch. **_DO NOT_** open one against `master` unless you are explicitly told to. All "unreleased" work happens in the `development` branch.
+
+Also make sure to check [Buffalo's contributing guidelines](https://github.com/gobuffalo/buffalo/blob/development/.github/CONTRIBUTING.md) before starting to contribute.