teams/rust-by-example: synchronize with GitHub

[pietroalbini]

This will create a new rust-by-example team in the rust-lang GitHub organization with steveklabnik and marioidival. Then we should configure the rust-by-example repo to authorize the team instead of individual members.

r? @steveklabnik

[steveklabnik]

r+! (I don't think we actually use bors here, right? Should I just merge?)

[Manishearth]

Yes

[pietroalbini]

Permissions tweaked on the rust-lang/rust-by-example repository to avoid using collaborators and instead give write access to the new team. @marioidival you should have received an automatic invitation to join the rust-lang org, please accept it to gain write access to the RBE repo back!