Add mandoc-manpage for chmod(1)

and mark it as finished in README.

One small rationale on the way the manpage is set up: Looking at
the coreutils manpage, it does not invite to be a quick reference
guide, whereas I wrote this manpage to be short and concise in regard
to the information the advanced user needs.
No one needs to explain what an octal number is. That's not part of
the scope of this manpage.
Also, nobody wants to read a block of text just to find out how
to build an octal mode string.

README

@@ -12,7 +12,7 @@ The following tools are implemented (* == finished):
 * cal             yes                             none
 * cat             yes                             none
   chgrp           no                              -h, -H, -L, -P
-  chmod           yes                             none
+* chmod           yes                             none
   chown           no                              -h, -H, -L, -P
   chroot          non-posix                       none
   cksum           yes                             none