README: Add brief description for manpages/apropos.

Trying to mimic the manpage of (GNU) `ls`, we don't conjugate the verb as
"downloads" or something else.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Merge pull request #905 from rbrito/manpage-apropos

README: Add brief description for manpages/apropos.