Add documentation for installing tools

[innovate-invent]

This bit of documentation should be front page and not buried in code in another repo:

If there is a need to execute actions that would require a live galaxy instance, such as adding workflows, setting quotas, adding more users, etc.
then place a file with that logic named post-start-actions.sh on the /export/ directory, it should have access to all environment variables
visible here.
The file needs to be executable (chmod a+x post-start-actions.sh)

https://github.com/galaxyproject/ansible-galaxy-extras/blob/36c5b7b7dc49c14a78a91d3f063d927262d1c251/templates/startup.sh.j2#L455-L458

Even better is a few lines down:
https://github.com/galaxyproject/ansible-galaxy-extras/blob/36c5b7b7dc49c14a78a91d3f063d927262d1c251/templates/startup.sh.j2#L467-L480

You can install tools by specifying a list of paths to ephemeris yml files using an environment variable:
ENV GALAXY_AUTO_UPDATE_TOOLS="/export/mytools1.yml /export/mytools2.yml" #Space or line seperated

Related to #498 but not duplicate