Fifth draft of 0.90 release notes

Warnings: better description of root permission and WiFi effects.

Known Issues: remove issue #35 (resolved, MeshMS is now encrypted in transit),
add issue #53 (new, unshare does not update file list).

Re-organise Copyright and Licensing section: copyright in first para, licensing
in second, and remove third para about developer agreements, because that
information is best presented on the Wiki.
Add Privacy Policy text

Copied from assets/helprivacy.html and heavily edited.