Allow prefilling of any fields of the login form using GET params (#181)

* Allow prefilling of any fields of the login form using GET params

* update readme

docs/prefill-login-form.md

@@ -0,0 +1,48 @@
+# Prefilling the Login Form
+
+In some cases you may wish to prefill/preset the login form fields when sharing a link to a Synapse Admin instance.
+This can be done by adding the following query parameters to the URL:
+
+* `username` - The username to prefill in the username field.
+* `server` - The server to prefill in the homeserver url field.
+
+The following query params will work only if the Synapse Admin is loaded from `localhost` or `127.0.0.1`:
+
+* `password` - The password to prefill in the password field (credentials auth). **NEVER** use this in production.
+* `accessToken` - The access token to prefill in the access token field (access token auth). **NEVER** use this in production.
+
+> **WARNING**: Never use the `password` or `accessToken` query parameters in production as they can be easily extracted
+from the URL. These are only meant for development purposes and local environments.
+
+
+## Examples
+
+### Production
+
+```bash
+https://admin.etke.cc?username=admin&server=https://matrix.example.com
+```
+
+This will open `Credentials` (username/password) login form with the username field prefilled with `admin` and the
+Homeserver URL field prefilled with `https://matrix.example.com`.
+
+### Development and Local environments
+
+**With Password**
+
+```bash
+http://localhost:8080?username=admin&server=https://matrix.example.com&password=secret
+```
+
+This will open `Credentials` (username/password) login form with the username field prefilled with `admin`, the
+Homeserver URL field prefilled with `https://matrix.example.com` and the password field prefilled with `secret`.
+
+
+**With Access Token**
+
+```bash
+http://localhost:8080?server=https://matrix.example.com&accessToken=secret
+```
+
+This will open `Access Token` login form with the Homeserver URL field prefilled with `https://matrix.example.com` and
+the access token field prefilled with `secret`.