Proposal: multiline should be documented inside REPL

I eventually found the `Ctrl-V` answer in StackOverflow, but not before I'd used the REPL for about 2 weeks thinking that multiline was broken in the REPL. Also, while StackOverflow told me to use `Ctrl-V` to enter multiline mode, it didn't mention to use that keycode to exit. So, as a Unix user, I hit Ctrl-D and the REPL quit. It would be nice if the REPL told people about `Ctrl-V` to enter and exit multiline mode. Also, it would be nice if it told people about `.help`.

CoffeeScript version 2.3.1
Node v10.4.0

Start by locating the CoffeeScript REPL's multiline-mode guidance and `.help` output. Update the in-REPL instructions to explain that Ctrl-V enters and exits multiline mode and to mention `.help`; done means users can discover both without external documentation.

Written by the indexing model from the issue text.