Get long package description for installed packages from installed files

Get long package description for installed packages from installed files

* doc/lispref/package.texi (Archive Web Server): New; document web
server interface.

* lisp/emacs-lisp/package.el (package--get-description): New; get long
description from installed files.
(describe-package-1): Use it, improve comments. No longer writing
NAME-readme.txt.

* test/lisp/emacs-lisp/package-tests.el:
(package-test-describe-package): There is now a description for an
installed package.
(package-test-describe-installed-multi-file-package): New test.