Add a note about plugin incompatibility after 1.2 > 2.0

**Environment**:

- CTFd Version/Commit: master

**What happened?**

I just tried CTFd and it looks pretty cool! After trying to install different plugins from https://github.com/CTFd/plugins the start failed with most of the listed plugins.

**What did you expect to happen?**

I expected a info of notice about the changes that happened during the move from 1.2 to 2.0 and that the incompatibility is a known issue. I did found a info in https://github.com/CTFd/CTFd/releases/tag/2.0.0 but it did took me a while to find it since I first search for the error messages/isssues.

I know that 3rd party plugins are not in your scope, but you're listing them in a plugin repo: https://github.com/CTFd/plugins. Maybe a note in this repo would be nice.