MDLSITE-2009 minor improvements to prerelease.sh and README.md

Two new commits here:

commit 6fd09b8af34e647ff4e3f2c1e0efb1c132d23751
Author: Sam Hemelryk <sam@moodle.com>
Date:   Mon Nov 18 10:12:23 2013 +1300

    MDLSITE-2009 tidied up the README.md file

commit 99a82f13e615aa3f9489cb45a5d65035d8fab7fe
Author: Sam Hemelryk <sam@moodle.com>
Date:   Mon Nov 18 11:41:07 2013 +1300

    MDLSITE-2009 prerelease: several minor improvements
    
    * Moved allbranches var to global space so that others can use it.
      Converted uses to the new global as appropriate.
    * Converted echo usage within reset_repo and show_changes to use output
      allowing their use within scripts in conjunction with --quiet.
    * Added branch existence checking and responsible output to reset_repo
      and show_changes so that errors are avoided when expected branches
      are missing.
    * Minor help formatting improvements.