Fixes https://github.com/thoughtbot/administrate/issues/1806
The "Getting started" section in the README is currently a duplicate of the one in the docs. They also provide different level of information, which can confuse readers who have read one but have questions solved by the other.
This MR removes this section from the README, and adds missing information to the one in the docs. This should help us maintain a single, unified, and reliable source of documentation.