Add README entry for GRIST_INCLUDE_CUSTOM_SCRIPT_URL variable (#906)

README.md

@@ -271,6 +271,7 @@ GRIST_HOST          | hostname to use when listening on a port.
 GRIST_HTTPS_PROXY   | if set, use this proxy for webhook payload delivery.
 GRIST_ID_PREFIX | for subdomains of form o-*, expect or produce o-${GRIST_ID_PREFIX}*.
 GRIST_IGNORE_SESSION | if set, Grist will not use a session for authentication.
+GRIST_INCLUDE_CUSTOM_SCRIPT_URL | if set, will load the referenced URL in a `<script>` tag on all app pages.
 GRIST_INST_DIR      | path to Grist instance configuration files, for Grist server.
 GRIST_LIST_PUBLIC_SITES | if set to true, sites shared with the public will be listed for anonymous users. Defaults to false.
 GRIST_MANAGED_WORKERS | if set, Grist can assume that if a url targeted at a doc worker returns a 404, that worker is gone