Add some instructions on configuring sendmail.

Usually, when I ran the Drush unit tests, I did so on development systems configured to deliver email locally only.  Recently, though, I ran them on a system where sendmail was not configured at all; this caused some test to fail due to mail delivery errors.

It might be nice to set up the unit tests to robustly subvert outgoing email.  The easiest way to disable email, without requiring custom modules or hooks (which would be too invasive for unit tests) is to set the php `sendmail_path` configuration variable.  I could not determine a good value to set this to that would work cross-platform, though, so in the end I decided to just add documentation that would allow the end user to fix the problem manually for their system, if necessary.