README: Update generally

README: Update generally

    * Reflect fact that project ships pages in sections 1-8.

    * Resync documentation of the basic Makefile targets with what they
      actually do at present.  Slighly simplify discussion.

    * Put the man page overlaps into a table.
      + Drop references to overlaps with the old a.out ld.so, long dead.

    * Update reference to Announce file to use current naming convention.
      + Send people looking for copyright information there rather than
        duplicating some of it here.

    * Direct reader to the top-level homepage instead of more deeply to the
      "Contributing" page.

    * Reflow paragraphs to 72 columns.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>