CONTRIBUTE cleanups and updates

CONTRIBUTE cleanups and updates

* CONTRIBUTE: Mention URLs and info nodes more consistently,
avoiding possibly-confusing punctuation adjacent to a URL, and
giving full shell commands for 'info'.  Start with a brief but
complete how-to, for people who want to get started right away.
Then briefly discuss how to join the development process in the
typical order.  Omit needless words.  Update some of the
now-obsolete file names, info node names, and quoting styles.
Better document emacs-NN branches and how they are merged.
* admin/notes/git-workflow: Change emacs-24 to emacs-25,
and trunk to master.  This file still needs work.