condensed rewrite of chapter 6.2 Versioning

Weeded out a possible contradiction where one might understand that all
versions prior to 1.0 are considered incompatible, but there was also a
note on preserving backward compatibility with 0.95
condensed rewrite of chapter 6.2 Versioning (step 2)

added possibility to allow for versions below 1.0 to be valid.
Merge pull request #42 from paul-esch/master

condensed rewrite chapter 6.2 Versioning