In the github world, it has become popular to follow Tom Preston-Werner's "semver" spec for version numbers.
From http://semver.org/ :
Given a version number MAJOR.MINOR.PATCH, increment the:
MAJOR version when you make incompatible API changes, MINOR version
when you add functionality in a backwards-compatible manner, and PATCH
version when you make backwards-compatible bug fixes. Additional
labels for pre-release and build metadata are available as extensions
to the MAJOR.MINOR.PATCH format.
