Update contributing guide to be explicit about creating a fork prior to cloning

[stacey-gammon]

Then other github newbies like me won't create and push branches directly to the Kibana repo without going through a pull request. Doh!

This is my first PR so please let me know if I am doing something wrong, or if anything can be improved. :)

[karmi]

Hi @stacey-gammon, we have found your signature in our records, but it seems like you have signed with a different e-mail than the one used in yout Git commit. Can you please add both of these e-mails into your Github profile (they can be hidden), so we can match your e-mails to your Github profile?

[stacey-gammon]

Done!

[ycombinator]

LGTM!