diff --git a/README.md b/README.md
index 08e9b299d8b7f6254bd50e0d4b6c47d4b8140932..ec104db9d4ef40a13c0768b68fa0d5ce762416bf 100644
--- a/README.md
+++ b/README.md
@@ -63,6 +63,14 @@ visitors.
 (Technically the # and @ in the URL fragment should probably be escaped, but in
 practice for legibility we bend the rules and include it verbatim)
 
+### Optional parameters
+
+https://matrix.to/#/#matrix:matrix.org?web-instance[element.io]=chat.mozilla.org
+
+- `client`, e.g. `client=im.fluffychat`, `client=element.io`
+- `web-instance[]`, e.g. `web-instance[element.io]=chat.mozilla.org`
+-  `via`, e.g. `via=mozilla.org`
+
 You can discuss matrix.to in
 [`#matrix.to:matrix.org`](https://matrix.to/#/#matrix.to:matrix.org)