Indicate deploy status of schema changes

[badboy]

Last week I was bitten by the fact that a merged schema does not mean it's also deployed and in use.

My suggestion:

• Document in the README that merge != deploy.
• Have a way to find out what is deployed (an endpoint giving the latest deployed commit id?)

[mreid-moz]

cc @whd

[acmiyaguchi]

https://protosaur.dev/mps-deploys/ is available for deployment status, but we currently don't have this information propagated publicly. Bug 1609738 may be relevant here.