Start rewrite in rfc2119 -style

[phadej]

Changes:

• made an enumerated list merging in some subsections
• yet each item has short name, for help referencing, and possible re-ordering
• haven't changed the wording

[phadej]

ping @hvr @dcoutts @bergmark

[phadej]

This also adds citation about semver as it's already in wiki. Supersedes https://github.com/haskell/pvp/pull/1

[glguy]

These changes make sense to me and seem well received on the mailing list thread https://groups.google.com/forum/#!topic/haskell-core-libraries/XAVb_CI-Oak