Add support for automatic Double Puppeting for all Mautrix bridges

docs/configuring-playbook-bridge-mautrix-whatsapp.md

@@ -10,4 +10,36 @@ Use the following playbook configuration:
 matrix_mautrix_whatsapp_enabled: true
 ```
 
+
+## Set up Double Puppeting
+
+If you'd like to use [Double Puppeting](https://github.com/tulir/mautrix-whatsapp/wiki/Authentication#replacing-whatsapp-accounts-matrix-puppet-with-matrix-account) (hint: you most likely do), you have 2 ways of going about it.
+
+### Method 1: automatically, by enabling Shared Secret Auth
+
+The bridge will automatically perform Double Puppeting if you enable [Shared Secret Auth](configuring-playbook-shared-secret-auth.md) for this playbook.
+
+This is the recommended way of setting up Double Puppeting, as it's easier to accomplish, works for all your users automatically, and has less of a chance of breaking in the future.
+
+### Method 2: manually, by asking each user to provide a working access token
+
+**Note**: This method for enabling Double Puppeting can be configured only after you've already set up bridging (see [Usage](#usage)).
+
+When using this method, **each user** that wishes to enable Double Puppeting needs to follow the following steps:
+
+- retrieve a Matrix access token for yourself. You can use the following command:
+
+```
+curl \
+--data '{"identifier": {"type": "m.id.user", "user": "YOUR_MATRIX_USERNAME" }, "password": "YOUR_MATRIX_PASSWORD", "type": "m.login.password", "device_id": "Mautrix-Whatsapp", "initial_device_display_name": "Mautrix-Whatsapp"}' \
+https://matrix.DOMAIN/_matrix/client/r0/login
+```
+
+- send the access token to the bot. Example: `login-matrix MATRIX_ACCESS_TOKEN_HERE`
+
+- make sure you don't log out the `Mautrix-Whatsapp` device some time in the future, as that would break the Double Puppeting feature
+
+
+## Usage
+
 You then need to start a chat with `@whatsappbot:YOUR_DOMAIN` (where `YOUR_DOMAIN` is your base domain, not the `matrix.` domain).