Create CONTRIBUTING.md

CONTRIBUTING.md

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+## Contributing code
+
+Any code that you contribute will be licensed under the GPLv3-license adopted by salmon. However, by contributing
+code to this project, you also extend permission for your contribution to be re-licensed under the BSD 3-clause 
+license (under which we anticipate Salmon will be released once existing GPL code can be removed).
+
+Code contributions should be made via pull requests.  Please make all PRs to the _develop_ branch 
+of the repository.  PRs made to the _master_ branch may be rejected if they cannot be cleanly rebased 
+on _develop_.  Before you make a PR, please check that:
+
+ * Your PR describes the purpose of your commit. Is it fixing a bug, adding functionality, etc.?
+ * Commit messages have been made using [*conventional commits*](https://www.conventionalcommits.org/en/v1.0.0/) — please format all of your commit messages as such.
+ * Any non-obvious code is documented (we don't yet have formal documentation guidelines yet, so use common sense)