Add README file and improve usage message

Nagios plugin to check lock file state
======================================

This plugin check lock file statue.

Usage
-----

  Usage : ./check_lock_file -l /path/to/lock.file [-o uid] [-w W] [-c C] [-d] [-h]

  Check lock file status

  Parameters :
    -l /path/to/lock.file   Path to lock file
    -o uid                  Valid lock file owner (optional)
    -w W                    Exit with WARNING status if lock file
                            has been created for more than W hours
    -c C                    Exit with CRITICAL status if lock file
                            has been created for more than C hours
    -d                      Enable debug mode
    -h                      Show this message

Copyright
---------

Copyright (c) 2014 Benjamin Renard

License
-------

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License version 2
as published by the Free Software Foundation.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.