Add documentation for using custom email domain

The [custom domain documentation](https://github.com/Azure/wordpress-linux-appservice/blob/7e593b214c040afbf2c9937fb365f4d870b9b425/WordPress/wordpress_migration_linux_appservices.md#configuring-custom-domain) only provides instructions for the app service. It would be great if there was additional documentation on changing the email domain so that the sender address is `DoNotReply@mydomain.com` instead of the default which looks something like `DoNotReply@a9ad25fe-cef3-43d3-bbe8-8b57eb2fce56.us1.azurecomm.net`.

These were the steps I took to achieve that:

1. Go to the *Email Communication Service* resource > **Settings** > **Provision domains**
2. Select **Add domain** > **Custom domain**
3. Follow prompts to add the domain name
- Note: This will create a new *Email Communication Services Domain* resource.
- Note: For proper domain verification, I had to use `@` instead of the domain for the TXT name.
4. Go to the *Communication Service* resource > **Email** > **Domains**
5. Disconnect the default domain: **Options (3 dots)** > **Disconnect**
6. Select **Connect domain**
7. Follow prompts to add the new *Email Communication Services Domain* resource
8. Go to the *App Service* resource > **Settings** > **Environment variables**
9. Edit the `WP_EMAIL_CONNECTION_STRING`
10. Replace `senderaddress=DoNotReply@xxxxxx.azurecomm.net` with `senderaddress=DoNotReply@mydomain.com`
11. Apply changes
12. Restart app service (?)

I can submit these steps as a PR if preferred.