Add note about Change-Id to setup instructions

This is an additional step that bloggers need to go through to
successfully write blog posts.

Change-Id: Ibb7e6cca3cfd6d645be01b45ee54ace3e17111c4
Reviewed-on: https://gerrit.causes.com/29636
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>