removed a reference to an Issue with installation on Mac OS, which is now Closed - https://github.com/michaelmcandrew/civicrm-buildkit-docker/issues/15

Actually I think we should put the 'other platforms' first & the more specific options at the end though

I didn't add in the ESR as I don't believe the policy  is clear enough to be documented but generally I understand that the discussion takes place on the ESR channel & PRs are not really encouraged against the branch - the CT would do a PR if they decide to backport or agree someone else would

Fix comma splice.  Make clearer reference to later section.

Clarified when the backtrace is displayed, i.e. calling &backtrace=1 does not trigger an immediate backtrace unless an error occurs.

Ref dev/core#86

Also:

- Clarify that PRs don't need to have associated tickets
- Clean up some language here and there

Add health warning about running Docker on OSX

There are usually a few branches to choose from when picking a base-branch.
This breaks out a table to differentiate them.

Added documentation on Michael McAndrews docker container.

If I disagree with using the "NFC" label, I'd like to be able to quickly
reference a link that explains the rationale and provides examples.

This makes it easier to reference these things from other pages.