Added clarification to docs about config for custom domains

[Descent098]

Added clarification about config for custom domains, specifically the case where your base URL on github pages has a custom domain.

For example, my github account pages repo ( descent098.github.io) links to kieranwood.ca, and because of that you need to specify both the custom domain name, and base URL to get it to work, because github will add it to https://kieranwood.ca/up

Example here:

...
status-website:
  cname: kieranwood.ca
  baseUrl: /up
...