JetBrains: Add compatibility note for M1 and old IDE versions
Created by: vdavid
Also added it under "Getting started", here: https://plugins.jetbrains.com/plugin/9682-sourcegraph
Test plan
(only docs change)
Created by: vdavid
Also added it under "Getting started", here: https://plugins.jetbrains.com/plugin/9682-sourcegraph
(only docs change)