Issue #2516842 by er.pushpinderrana, thejacer87, jhodgdon: UPGRADE.txt should explain where to find release notes

Issue #2507911 by naveenvalecha, mikebell_, dylanf, deepakaryan1988, kfitz, jhodgdon, webchick, Charles Belov: Mention location of settings.php in UPGRADE.txt

Issue #1167996 by rocket_nova, Sheldon Rampton: Fixed UPGRADE.txt needs to say all D6 contrib modules need to be upgraded to their latest D6 versions before the D6->D7 update.

Issue #1040810 by Tor Arne Thune, jhodgdon: Fixed UPGRADE.txt describes erroneous step and should mention release notes.

- Patch #1029346 by David_Rothstein: Changed INSTALL.txt and UPGRADE.txt should mention that you can download Drupal as a .zip file also.

- Patch #295255 by yettyn, jhodgdon, sun, effulgentsia, nirbhasa: clean-up the upgrade path: UPGRADE.txt.

- Patch #865072 by bleen18, jhodgdon: Bartik is default theme now ... documentation & tests need to be changed.

#450540 by mcrittenden: Remove reference to Stark in upgrade documentation, since it only appears in D7.

#67234 by Ralf, Dave Reid, David_Rothstein, Rob Loach, dww, et al: Added a permission to update.php.

- Patch #570572 by winston, David_Rothstein | Shai, Xano, alexanderpas, emmajane, kazar: changed label for user/1 account from 'administrator' to 'site maintenance account'.

- Patch #430342 by kkaefer et al: usability improvement: changed 'site maintenance' to 'maintenance mode'.

#206281 by keith.smith: document that people should look into the new system requirements when they upgrade

#175876 by hazexp, webernet, keith.smith: unify module/theme/translation download links and include more pointers to these locations

#152967 by keith.smith, webchick, Morbus Iff and KentByte: update the upgrade instructions for Drupal 6

#170638 by JirkaRybka and chx: move update access variable to settings.php, so we can check whether it is wide open, and we have one place for settings

   + reorganized ?q=admin/settings
   + deprecated the _settings hook

- Created separate upgrade instructions.  Will hopefully make it easier to spot upgrade isntructions.