The WHATWG PR template should have a reference to Stages

[mfreed7]

At a recent joint WHATWG/CSSWG/OpenUI meeting, some concerns were raised around standards positions and early shipping of features. The concern was that marking a standards position "positive" unblocks shipment of features, perhaps before all details are worked out. While there are other safeguards against that, e.g. editor review and approval of the PR, it seems to make sense to bring the WHATWG stages process to the WHATWG PR template. For example, there seem to be two options to get a PR landed: 1) get standards positions from two implementers, or 2) get the proposal to stage 3.

Can we update the template accordingly?