Add an explanation of the labels

[carols10cents]

As requested @matklad! ❤️ Are there any questions you have about the labels that remain unanswered?

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[carols10cents]

r? @matklad

[matklad]

Wow, that is much more thorough explanations than I had expected! Thanks a lot @carols10cents , I'be definitely learned more about our issues now ❣️

@bors r+

[bors]

📌 Commit 0cfe635 has been approved by matklad

[bors]

⌛ Testing commit 0cfe635 with merge 36ddeff...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: matklad
Pushing 36ddeff to master...

[matklad]

@carols10cents what does E-mentor mean? I can think of at least to explanations:

1. an issue with an actual live mentor, whom you can ping on IRC, ask questions, etc.
2. an issue with some instructions on how to fix it (like, where's the relevant code, where are the tests, what should be changed).

Specifically, I've just created #4563 and I think it's mentor in the 2) sense, but not in the 1), so I don't know if I should apply the label :)

[carols10cents]

@matklad to me, it means at least 2) and optionally 1) if 2) is insufficient :) I don't know if my interpretation is common though!