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: improve; add explicit web references, move some info from
admin/notes/* here.
* INSTALL.REPO: You can't "just run make" after a clean checkout.
* admin/notes/commits: deleted; merged into ./CONTRIBUTE
* admin/notes/repo: move commit, branch info into ./CONTRIBUTE