Document how OG handles access and permissions

[pfrenssen]

Fixes #510.

[MPParsley]

Great writeup thanks!

Sometimes we write OG sometimes Organic Groups, I'd try to be consistent and use Organic Group everywhere but I'll let you decide.

We should probably also reference this document from the README.md

[amitaibu]

Sometimes we write OG sometimes Organic Groups

I usually write the first occurrence in the doc as "Organic Groups (OG), ...", and then continue with OG

[pfrenssen]

Adding to the milestone since this is closely related to the work done to OG Access in the upcoming release, it would be good to also include this documentation along with the changes.

[pfrenssen]

This has already been approved, but changes were made afterwards. @amitaibu or @MPParsley do these changes look OK?

[amitaibu]

Yeah, looks great, thanks!

[MPParsley]

Let's merge this. The phpcs issues are not related to this PR.

[MPParsley]

Error in Travis will be fixed in #720.

1) Drupal\Tests\og\Functional\GroupSubscribeTest::testSubscribeAccess
Behat\Mink\Exception\ExpectationException: Current response status code is 404, but 500 expected.

[pfrenssen]

Thanks! Good job preparing the release, unfortunately I don't have the availability to assist at the moment.