Semantic Versioning 2.0.0

Semver v2 was published in 2013. The only file changed between v1 and v2 was semver.md. Here's the diff: https://github.com/semver/semver/compare/v1.0.0...v2.0.0#fil...

In addition to expanding and clarifying some definitions, it added a paragraph about deprecating features and a sentence about size limits on the semver string (there are none, but 255 characters should be enough for anyone).