* [Diagnostics] Turn educational notes on-by-default
* [Diagnostics] Only include educational notes in printed output if -print-educational-notes is passed
* Make -print-educational-notes a driver option
* [Diagnostics] Issue a printed remark if educational notes are available, but disabled
* [docs] Update educational notes documentation and add a contributing guide
* [Diagnostics] Cleanup PrintingDiagnosticConsumer handling of edu notes
* Revert "[Diagnostics] Issue a printed remark if educational notes are available, but disabled"
For now, don't notify users if edu notes are available but disabled. This decision can be reevaluated later.
A handful of people have recently asked me if such a thing existed;
now it does. There's certainly more that can be added here, but I
wanted to dump a bunch of the information in my brain into doc form.
