Updating README and testReleaseHelp to include 'rehash' for csh users

Sadly, csh users who set their environment first and build second (as
we now require in the quickstart world) need to do a rehash in order
to update their path to see 'chpl'.  Added a step for them in the README
and updated the testReleaseHelp script to do the same thing.
Merge pull request #300 from bradcray/fixCshQuickStartWithRehash

Updating README and testReleaseHelp to include 'rehash' for csh users

[not reviewed, though concept approved by hilde, tvandoren]

Sadly, csh users who set their environment first and build second (as
we now require in the quickstart world) need to do a rehash in order
to update their path to see 'chpl'.  Added a step for them in the README
and updated the testReleaseHelp script to do the same thing.