Merge pull request openedx#1211 from edx/ned/simpler-install-invocation

No need for $OPENEDX_RELEASE on install_stack.sh

Merge pull request openedx#1211 from edx/ned/simpler-install-invocation

No need for $OPENEDX_RELEASE on install_stack.sh

Adds information about the new ability to use a CDN for course assets, (

openedx#765)

and how to configure what the CDN information is so courses specify the
right URLs.
(cherry picked from commit f00209a)

Fixes DOC-3041 (openedx#1111)

Dogwood.3 release note

Instructions for how to upgrade to Dogwood point releases

Updates Git tag

Merge pull request openedx#858 from edx/ahodges/dogwood_IRC

None of the features require a step to enable AFAICT

Merge pull request openedx#785 from edx/ned/dogwood-install-docs

Dogwood install docs

Merge pull request openedx#724 from edx/replace-irc-with-slack

Replace IRC with Slack