Add git and/or github to help menu

[rickr]

We want to add some instructions to new users, on how they can familiarize themselves with version control, Git, and Github.

Right now, the bot sends a welcome DM on Slack when a new user joins, and provides them some options on what they can learn for a language... Python, Ruby, etc.

We'd like to link them to some useful training on version control, so they can get started on self-taught courses right away. This can be as simple as pushing them to Github's current training offerings, as an MVP.

[hollomancer]

https://github.com/OperationCode/operationcode_bot/tree/master/views/help_menu and https://github.com/OperationCode/operationcode_bot/tree/master/lib are good places to start here.