doc: move README into manual

doc: move README into manual

* README: Move contents to new file doc/gnulib-readme.texi.
Replace with a one-line summary.
* doc/gnulib.texi (Brief Overview): New section,
with old intro preface.  Include gnulib-readme.texi for contents.
(Philosophy): Rename from "Introduction", since this
section no longer introduces the rest.  Write a new preface.
* doc/gnulib-readme.texi: New file, with the old contents of
README texinfo-ized.  This way, the README info appears
in the online and printed manual.