Set policy for versioning

[apulsipher]

We need to set a policy for updating the version number for epiworldR. At the moment, we are keeping the version number the same as the C++ library, but that may not reflect updates to the R package that aren't tied to underlying changes in the C++ library.

[apulsipher]

As of #66, we've begun the policy of keeping the first three version numbers consistent with the C++ library (MAJOR.MINOR-PATCH) but we've added a fourth version number to reflect changes to the R package only. So the new format is MAJOR.MINOR-PATCH.R_PATCH. While the policy is set, before we close this issue we should note that in the docs somewhere.