Instructions on upgrading plugins for Elgg 1.9

This is WIP. Do not merge.

We need to decide:
 - Do we want to include plugin upgrade instructions to the rst docs?
 - How detailed information do we want to include?
 - Is ```guides/``` the correct place for this doc?

TODO:
 - [x] Move to `guides/upgrading`
 - [x] Add links to example commits where the changes were made (e.g. for community site plugins).
 - [x] remove extra commit from PR