Update readme.

[rosiel]

GitHub Issue: none - just update readme, it's out of date.

What's new?

A in-depth description of the changes made by this PR. Technical details and
possible side effects.

• Changes x feature to such that y
• Added x
• Removed y
• Does this change require documentation to be updated?
• Does this change add any new dependencies?
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)?
• Could this change impact execution of existing code?

How should this be tested?

• Remove "8"
• update link to playbook role
• don't mention the outdated 'update submodules' business
• link to isle as well as playbook

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

Tag (@ mention) interested parties or, if unsure, @Islandora/8-x-committers