git: consolidate and simplify .gitignore.

* bin/.gitignore, build-aux/.gitignore: Consolidate into...
* .gitignore: ...here.  Simplify.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: don't force distribution of README.md.

* Makefile.am (EXTRA_DIST): Don't force distribution of README.md.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
README: INSTALL is only available on release branch.

* README.md: Fix url for Automake installed INSTALL file to point
at release branch, where it is checked in.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>