[support] Avoiding a drush / drupal version mismatch

[Dan Kegel]

I just wasted a few hours trying to track down the error

Table 'drupal.variable' doesn't exist

when doing drush si.  Turns out that the releases of drush marked 6.x at
https://drupal.org/node/97249/release?api_version%5B%5D=87
work fine with Drupal 6, but the releases at
https://github.com/drush-ops/drush/releases
all seem to exhibit the bug.  I guess the github project
releases simply aren't meant to support Drupal 6.

So, if you're trying to use drush with drupal 6, best to
use the old download page, not the one at github.

Probably this was obvious to everyone else, but it was a surprise to me.