Create CONTRIBUTING.md

Author: serverco

CONTRIBUTING.md

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+# How to contribute
+
+If you are happy writing in bash, please create a PR for any changes you'd like to see included (or bug fixes).  
+
+If you aren't happy writing in bash, please open an issue with as much detail as possible about the issue or what you'd like to see added / improved.
+
+## Submitting changes
+
+Please update the 'revision history' and version number at the top of the code (without this I can't easily do a merge)
+
+## Coding conventions
+
+Please see the guidelines at https://github.com/srvrco/getssl/wiki/Bash-Style-guide
+
+## Testing
+
+Please test with [shellcheck](https://github.com/koalaman/shellcheck), although this will also be tested on github ( via travis) on all PRs. 
+
+Please remember that the system is used across a wide range of platforms, so if you have access to multiple operating systems, please test on all.
+
+
+Thanks :)