Update readme with instructions on approving local certs

[zanematthew]

While developing with SSL/HTTPs locally, it can be rather annoying to have the red "x", and browser flags that the cert is not approved.

Update the readme with the steps (currently specific to OS X) for adding the certs.

[zanematthew]

I've noticed that I updated the docs on the my other pull request, #20, let me know if you want a separate pull request for this issue, also let me know;

1. If you are you actively accepting pull request
2. If you will/would accept this pull request