Update README.rst for occ updater

[marcos-mendez]

Instruction to upgrade NC in bash

[JedMeister]

Sorry for such a ridiculously late feedback... :(

The idea behind this is awesome and I would love to include much better docs for updating. However I have some reservations/ideas on the implementation.

• I would prefer moving the specifics of the Nextcloud update process to a separate doc in a (new) docs/ dir - e.g. docs/update.md (or similar; not attached to the name).
• Replace the current link in the readme to the new doc & include upstream docs link in docs/update.md
• In an earlier PR (Update README.rst #22 - which I've just closed) you note the potential need to update PHP - that should also be covered in the new docs/update.md too.
• Recommend use of the TurnKey helper scripts where/if relevant:
  • tkl-upgrade-php
  • turnkey-php
  • turnkey-occ