Add info about bulk-updating instances to README.

Created by: itsjeyd

Part of the scope of OC-2608.

The example script was used successfully to bulk-upgrade instances from Ficus.1 to Ficus.3.

TODO (as part of future tasks involving instance upgrades):

• Turn the script for upgrading instances into a management command.
  • Rework step 3 and 4 so that they will block until the low priority huey queue is empty.
  • Parameterize the management command on:
    • The main tag name to use (ficus3-redeployment in the script). We can derive success and failure tags from that name automatically.
    • The source release to use for selecting instances (open-release/ficus.1 in the script).
    • The target release to upgrade to (open-release/ficus.3 in the script).
  • To avoid the possibility of being left with some instances that failed to upgrade after the command finishes, modify step 4 to iterate until the set of instances currently tagged with failure_tag is empty.

Reviewers

• @bradenmacdonald