added contribution doc

CONTRIBUTING.md

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+# Contributing to haproxy-boshrelease
+
+## Contributor License Agreement
+
+Follow these steps to make a contribution to any of CF open source repositories:
+
+1. Ensure that you have completed our CLA Agreement for
+   [individuals](http://cloudfoundry.org/pdfs/CFF_Individual_CLA.pdf) or
+   [corporations](http://cloudfoundry.org/pdfs/CFF_Corporate_CLA.pdf).
+
+1. Set your name and email (these should match the information on your submitted CLA)
+
+        git config --global user.name "Firstname Lastname"
+        git config --global user.email "[email protected]"
+
+1. If your company has signed a Corporate CLA, but sure to make the membership in your company's github organization public
+
+
+## Development
+
+The easiest way to get your contribution included is to submit a PR on a feature branch against master.
+
+It may help to open an issue prior to working on the feature, so that any discussions about how to best
+solve the problem can happen ahead of time.