Copy semantic versioning from metasploit-credential

MSP-11157

Dedupe with metasploit-version once it is released.
Add README and CONTRIBUTING.md

[ci skip]
MSP-11157

Add README and CONTRIBUTING that explains how to update the semantic
version.  Adapted from metasploit-credential's version of the files.
Merge pull request #2 from rapid7/chore/MSP-11157/semantic-versioning

Semantic Versioning