Revise contributing guide

There's at least an error with the component thing in merge requests. Might have other issues, too.

changed milestone to %GTK 0.1.0-beta3

added Document T: task labels

mentioned in commit 9554261f

mentioned in merge request !96 (merged)

mentioned in commit python-briar-wrapper@39e884e1

mentioned in merge request python-briar-wrapper!20 (merged)

assigned to @nicoalt

closed with merge request !96 (merged)