Text about major version change is neaten

See https://github.com/mojombo/semver/issues/130
Hmm, this wording is confusing to me. What if we focus on the aspect of a "release". After all, if you don't release it, the version doesn't really matter.

Something like:

```
Major version X (X.y.z | X > 0) MUST be incremented for a release of the API that contains any backward incompatible changes.
```