Adding releasenotes to 7.3 now

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1499

Reported by Andreas Lange, Roxen IS lange@roxen.com

To avoid having the story with 7.2 releasenotes comming RSN for a few weeks repeated, why not adding a releasenotes file to the 7.3 CVS now?

Small comments on major updates could be added once in a while, more descriptive text and examples could be filled in closer to the release.

Perhaps it could be useful to have some headlines from the beginning, ie 'new functions', 'compatibility changes', 'optimizations', 'modules' etc...