Update instructions in README

[JGallardo]

Followed instructions from gatsbyjs/gatsby#2836
Where you said you had no issues, but After changing my Procfile and package.json as you instructed, still issues.

Your instructions are not clear on the README if one were to follow from here. Could you add more details about what to clone, etc. Recently when starting creating a gatsby blog and making tweaks you suggested I get the following errors in my logs.
Build failed -- check your build output

Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch

Process exited with status 137

Could you please add some more details about how to get this working on Heroku.