[Documentation] Please document how patches.crates-io works

[sneakywombat]

In the docs, there is a reference:

reindeer/docs/MANUAL.md

Lines 75 to 76 in f2572ae

	there. See [below](#Local-Patches) for details on how to maintain local
	changes.

But when following the link:

reindeer/docs/MANUAL.md

Lines 144 to 146 in f2572ae

	## Local Patches
	(TODO)

It would be wonderful to have this populated with some information. thanks!! reindeer rocks!