Adds documentation on using a GHE endpoint

[cmcculloh-kr]

Addresses #395

[olleolleolle]

Thank you for taking the time to edit the documentation!

There are some formatting issues with the commit, do you want to fix them, or should I carry this PR for you?

Things we can improve:

• use straight double quotes " to denote quoting in the shell command invocation
• use line-continuation, to make it fit

Example:

github_changelog_generator --github-site="https://github.yoursite.com" \
                           --github-api="https://github.yoursite.com/api/v3/"

[olleolleolle]

Thanks, @CormacMcCarthy!