Move user-level information from CONTRIBUTE to doc/emacs/trouble.texi

Fixes bug#19299

* CONTRIBUTE: Move user-level information to doc/emacs/trouble.texi
  (commit messages): new, gathered from comments on emacs-devel
  (Changelog notes): add reference to GNU coding standards section 5.2;
  doc 'present tense', bug fix format
  (branches): freeze announcements are made on info-gnu-emacs mailing
  list
  (git vs rename): new

* doc/emacs/trouble.texi: Move user-level information from CONTRIBUTE here

* lisp/startup.el (fancy-about-text): change buttons for etc/CONTRIBUTE
  to (info "(emacs)Contributing")
* CONTRIBUTE: add Savannah url, cleanup announcing freeze.
resolve ChangeLog merge conflict