Documents the assumptions on plugins versioning

[simonedavico]

We need to have some assumptions on the format of the versions, if we want to recognise version ranges.

We decided to support semantic versioning to recognise version ranges, since it's an established and commonly used versioning format.

For plugins of S.U.T.s that don't use the semver format, we allow the specification of an arbitrary version string. Nevertheless, we don't provide version ranges support for versioning formats different from server.

Refer also to: #6 (comment)