Document milestone usage in DEVELOPMENT.md

[handrews]

Splitting off from #3509, I've been using the milestones as follows:

• Issues go in the oldest open milestone to which they are relevant, if it seems likely that they could get addressed. This usually means either someone has volunteerd (and the issue is assigned to them) or there's an agreement in a TDC call or in response to the review label that it should be done
• PRs go in the milestone to which they are targeted

Before we publish a release, we should migrate any unfinished issues to the next milestone, or decide to take them out of active consideration (no milestone)