Merge pull request #2354 from peastman/support

Added CONTRIBUTING and SUPPORT files

CONTRIBUTING.md

+## How to Contribute to OpenMM Development
+
+We welcome anyone who wants to contribute to the project, whether by adding a feature,
+fixing a bug, or improving documentation.  The process is quite simple.
+
+First, it is always best to begin by opening an issue on Github that describes the change you
+want to make.  This gives everyone a chance to discuss it before you put in a lot of work.
+For bug fixes, we will confirm that the behavior is actually a bug and that the proposed fix
+is correct.  For new features, we will decide whether the proposed feature is something we
+want and discuss possible designs for it.
+
+Once everyone is in agreement, the next step is to
+[create a pull request](https://help.github.com/en/articles/about-pull-requests) with the code changes.
+For larger features, feel free to create the pull request even before the implementaton is
+finished so as to get early feedback on the code.  When doing this, put the letters "WIP" at
+the start of the title of the pull request to indicate it is still a work in progress.
+
+For new features, consult the [New Feature Checklist](https://github.com/pandegroup/openmm/wiki/Checklist-when-adding-a-new-feature),
+which lists various items that need to be included before the feature can be merged (documentation,
+tests, serialization, support for all APIs, etc.).  Not every item is necessarily applicable to
+every new feature, but usually at least some of them are.
+
+The core developers will review the pull request and may suggest changes.  Simply push the
+changes to the branch that is being pulled from, and they will automatically be added to the
+pull request.  In addition, the full test suite is automatically run on every pull request,
+and rerun every time a change is added.  Once the tests are passing and everyone is satisfied
+with the code, the pull request will be merged.  Congratulations on a succesful contribution!
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SUPPORT.md

+## How to Get Support for OpenMM
+
+There are two main venues for getting support for OpenMM: the [discussion forum](https://simtk.org/forums/viewforum.php?f=161)
+and the [Github repository](https://github.com/pandegroup/openmm).  There is some overlap
+between the two, but generally speaking the forum is for user oriented issues while the
+repository is for developer oriented issues.  If you have a question about how to use OpenMM
+(including writing programs that access it through its public API), post on the forum.  If
+you want to suggest a change to the code, or if you think you have found a bug,
+open an issue on Github.  The core developers monitor both, so don't worry if you aren't
+sure which one is most appropriate for your question.  We will see it either way.
+
+You also may want to consult the [documentation](http://docs.openmm.org/).  It is quite
+thorough, and you may be able to find the answer to your question.
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