docs: add FAQ

docs/SUMMARY.md

@@ -10,3 +10,4 @@
     - [NixOS](deploying/nixos.md)
 - [TURN](turn.md)
 - [Appservices](appservices.md)
+- [FAQ](faq.md)

docs/faq.md

@@ -0,0 +1,25 @@
+# FAQ
+
+Here are some of the most frequently asked questions about Conduit, and their answers.
+
+## Why do I get a `M_INCOMPATIBLE_ROOM_VERSION` error when trying to join some rooms?
+
+Conduit doesn't support room versions 1 and 2 at all, and doesn't properly support versions 3-5 currently. You can track the progress of adding support [here](https://gitlab.com/famedly/conduit/-/issues/433).
+
+## How do I setup sliding sync?
+
+You need to add a `org.matrix.msc3575.proxy` field to your `.well-known/matrix/client` response which points to Conduit. Here is an example:
+```json
+{
+  "m.homeserver": {
+    "base_url": "https://matrix.example.org"
+  },
+  "org.matrix.msc3575.proxy": {
+    "url": "https://matrix.example.org"
+  }
+}
+```
+
+## Can I migrate from Synapse to Conduit?
+
+Not really. You can reuse the domain of your current server with Conduit, but you have to leave all federated rooms first.