More clear instructions on setting up a custom domain

[eyalatiya]

The docs aren't clear that cname.vercel-dns.com should be replace with {my-domain}.vercel-dns.com.
At least it didn't seem to work if I don't do this.

[fmerian]

@eyalatiya Hello there! Thanks for the report!

We just updated the dashboard settings with clearer instructions to help you set up your custom domain, and related docs (#302).

Please let us know if it works!