Make implicit knowledge more explicit in getting_started doc

Add more clear instructions on how to acquire the oh-mainline source
code.

Closes ticket https://openhatch.org/bugs/issue101
I see that the original code suggests an https fork; is that preferable to ssh? Do we have any resources dedicated to ssh+git? Obviously this adds complication so I'm +0 to changing this, just figured it deserved a moment of thought.