Explain delegation more clearly

[edunham]

Confusion happened in servo/rust-harfbuzz#63 because Homu failed to explain what "approval" meant in the current context.

196 11:17:25 < edunham> jack: ok, how should we word homu's delegate message... "@NAME, @NAME has delegated reviewing this PR to you! Please review the code then approve it 
                        with '@bors-servo r=NAME' once you think it's ready to merge"?
196 11:18:31 < vlad> edunham: that would have made it clearer to me
196 11:18:36 < vlad> it's too bad there's no way to disable manual merges
196 11:20:30 < jdm_> edunham: delegating can also be used after the review is complete and some minor changes are still required; maybe "@DELEGATE_TARGET, @DELEGATE_SOURCE 
                     has granted you the ability to merge this PR! When the code is reviewed and ready to merge, please approve it with `@bors-servo r=NAME_OF_REVIEWER`."
196 11:21:05 < jdm_> we could conceivably use the name of the assignee instead of NAME_OF_REVIEWER
196 11:21:54 < jdm_> or we could check if the PR author == the delegate target, and suggest the name of the delegate source in that case
196 11:21:59 < jdm_> that seems like a reasonable heuristic

I'm not totally sure whether assignee is the right person to r=; is there a better value available in the state at delegation time that I should be using instead?

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This change is

[cbrewster]

We typically don't use the @ in r=

[Manishearth]

We use delegation for both telling people to r=reviewer and getting others involved in review. If PR author==delegatee, suggest r=delegator, else suggest r+

[edunham]

This has always been more of an issue than a viable PR so I've booted it over to #100 to clear the queue out