Include a list of keyboard shortcuts in help text

[wentasah]

This makes it easier for people to discover the keyboard shortcuts supported by julia-repl-mode. With this change, the help of julia-repl-mode (C-h f) as well as the list of activate modes (C-h m) shows the following list:

key             binding
---             -------

C-c             Prefix Command
<C-return>      julia-repl-send-line

C-c C-a         julia-repl-activate-parent
C-c C-b         julia-repl-send-buffer
C-c C-c         julia-repl-send-region-or-line
C-c C-d         julia-repl-doc
C-c C-e         julia-repl-edit
C-c C-l         julia-repl-list-methods
C-c RET         julia-repl-macroexpand
C-c C-p         julia-repl-cd
C-c C-s         julia-repl-prompt-set-inferior-buffer-name-suffix
C-c C-t         julia-repl-includet-buffer
C-c C-v         julia-repl-prompt-set-executable-key
C-c C-z         julia-repl

[tpapp]

Thanks!