Include instructions for using a custom domain

[gitname]

Currently, the tutorial does not contain information about using a custom domain. Some users of the tutorial have expressed a desire for the tutorial to include such information.

Here is an example Issue related to the CNAME file getting deleted whenever the developer would deploy his or her app: #89

The instructions about using a custom domain will mention:

• The CNAME file
• Any necessary configuration via the GitHub UI (e.g. modifying repository settings) (I don't know whether any is necessary)

References:

• https://docs.github.com/en/pages/configuring-a-custom-domain-for-your-github-pages-site