bug#37414: 26.3; (emacs) `Contributing': no `etc/CONTRIBUTE'

[Drew Adams]

> You have an old manual, from an older Emacs version.

emacs -Q for Emacs 26.3 says this in `(emacs) Contributing':

  If you would like to make more extensive contributions, see the
  'CONTRIBUTE' file in the Emacs distribution for information on
  how to be an Emacs developer.

> > Where is a user on MS Windows supposed to find file `CONTRIBUTE'?
> 
> In the source tree, as the contents of that file has changed and makes
> no sense without access to the sources.

No idea what "the source tree" means here.  That node tells
users to look for file CONTRIBUTE "in the Emacs distribution".
Where is it in the Emacs distribution?

The same node also says:

  It is important to write your patch based on the current
  working version.

But there's no spec of what constitutes a "patch" - what
to include in it or what form it takes.  Nothing about the
form of a patch file, change log etc.  Is such info in the
missing (?) CONTRIBUTE file?  If not, where is it?