Define support policy for OpenMM versions

[ijpulidos]

We probably need to define a policy in terms of what OpenMM versions we support. I am not sure OpenMM follows semantic versioning or any versioning system, but we should probably try to come up with a reasonable policy of what versions we want to support from OpenMM, assuming OpenMM uses MAJOR.MINOR.PATCH convention (which it does so far).

[mikemhenry]

This is a good question, OpenMM doesn't really follow semantic versioning (breaking API changes happen without MAJOR bump). I propose that we support the current release and at a best effort support the n-1 release.