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
For a newcomer contributor, I think offering one option rather than a choice between 2 options (ssh AND https) is more optimal for their experience.

https is less secure. 

ssh connection is the most secure and requires the public key generation from the local laptop, as documented here: https://help.github.com/articles/which-remote-url-should-i-use The public key generation is an extra step. So instead, we should recommend newcomers to use https to get this source code.