Documentation enhancements (#73)

docs/features/bridge-integration.md

@@ -1 +1,28 @@
-To be documented
+# Bridge Integration
+To help natural bridge integration into the regular usage of a Matrix client, mxisd provides a way for bridge to reply
+to 3PID queries if no mapping was found, allowing seamless bridging to a network.
+
+This is performed by implementing a specific endpoint on the bridge to map a 3PID lookup to a virtual user.
+
+**NOTE**: This document is incomplete and might be misleading. In doubt, come in our Matrix room.  
+You can also look at our [Email Bridge README](https://github.com/kamax-io/matrix-appservice-email#mxisd) for an example
+of working configuration.
+
+## Configuration
+```yaml
+lookup.recursive.bridge.enabled: <boolean>
+lookup.recursive.bridge.recursiveOnly: <boolean>
+lookup.recursive.bridge.server: <URL to the bridge endpoint for all 3PID medium>
+lookup.recursive.bridge.mappings:
+  <3PID MEDIUM HERE>: <URL to dedicated bridge for that medium>
+
+```
+
+## Integration
+Implement a simplified version of the [Identity service single lookup endpoint](https://kamax.io/matrix/api/identity_service/unstable.html#get-matrix-identity-api-v1-lookup)
+with only the following parameters needed:
+- `address`
+- `medium`
+- `mxid`
+
+Or an empty object if no resolution exists or desired.