docs: Add Docker Hub authentication example

Author: euanh

Sources/swift-container-plugin/Documentation.docc/authentication.md

@@ -6,12 +6,34 @@ Configure the plugin to authenticate to your container registry.
 
 Many container registries require authentication in order to push images, or even pull them.
 The plugin reads your registry credentials from a `.netrc` file in your home directory.
-Add a record into the `.netrc` file for each registry you use, the plugin uses the authentication by the registry you choose.
+Add a record into the `.netrc` file for each registry you use.
+The plugin chooses the correct record based on the hostname of the registry's authentication server.
+
+> For some registries, such as Docker Hub [(see example)](<doc:#Docker-Hub>), the authentication server hostname might not be the same as the registry hostname you use when pushing and pulling images.
 
 The following example shows placeholder values for the registry `registry.example.com`:
 
 ```
 machine registry.example.com
   login myuser
   password mypassword
-```
+```
+
+The following examples show how to set up the plugin for some popular registry providers.
+
+### Docker Hub
+
+> Don't use your Docker Hub account password to push and pull images.
+> Personal Access Tokens have restricted privileges and you can create a separate token for each integration you use, which you can monitor independently and revoke at any time.
+
+1. Log into Docker Hub and [generate a Personal Access Token](https://docs.docker.com/security/for-developers/access-tokens/) for Swift Container Plugin.
+
+2. **Set the token's access permissions to *Read & Write*.**
+
+3. Copy the token and add it, together with your Docker ID, to your `.netrc` file under the machine name `auth.docker.io`:
+
+```
+machine auth.docker.io
+  login mydockerid
+  password dckr_pat_B3FwrU...
+```