Documentation update for Git integration

[ThomasThelen]

Related to issue #144. This is a small PR adding a few notes about importing Git repositories into a Tale. I was inclined to add a small section to the documentation area that talks about adding Data to the Tale since its very related to that topic. I added the actual documentation for using it under the 'Actions Menu' section since it's where the feature is exposed.

[craig-willis]

Looks good. A few comments:

• We should probably include a section in "Creating New Tales" describing the "Create Tale from Git Repository" option.
• We could include a description of what's happening for more advanced users. (Git init, git remote add, git checkout) and note that they can do what we do via CLI if desired.
• At some point we could expand to a section on Git support in general, including how it works with IDE integrations (e.g., jupyterlab-git, Rstudio, etc)

[ThomasThelen]

Nice catches. I updated the documentation to include creating Tales with Git repositories. Right now for users to use the Jupyter Hub Git extension they need to have a few packages installed; this can of course be done with repo2docker config files but might be better suited to go in the buildpack? It might be a hassle for a user to go through the process of installing the packages after already creating the Tale.

If we don't want to do that, I can definitely add some documentation for creating the config files to use the extension.