Fill README of some plugins. Closes GH-747. Closes GH-749.
schedule: Move the whole deletion of an event in the same critical section.
INSTALL.md: add installing requirements under pip.

It seems that pip ignores `requirements.txt` unless it's installed separately.
INSTALL: install requirements with pip before Limnoria itself.

[SKIP CI]
Merge pull request #753 from Mkaysi/patch-1

INSTALL.md: add installing requirements under pip.