Cover 'notes' feature

[ben]

It was removed in #163 because the xrefs were breaking things, but since it's a feature, it could be useful to cover it. The last version of the deleted section can be found in commit 1c03a04.

[phil-blain]

Apart from using them for CI annotations, notes are also useful for mailing-list oriented projects because they can be used by git format-patch, which can write them in the outputed patches as "timely commentaries" (also see 8th paragraph of the "Sending your patches" section of SubmittingPatches ("You often want to add additional explanation about the patch...")

[KZiemian]

What was the problem with xrefs? And how is a status of notes today?