doc: add "whiteboard" instructions to README

Had a hiccup installing whiteboard today. Expected it to "just work",
 but there are a few manual steps I needed to do. I wrote those out in
 the README, since we don't have more structured nextcloud docs yet.

README.md

@@ -399,6 +399,42 @@ docker exec -u www-data $(docker ps -f name=foo_com_app -q) ./occ preview:pre-ge
 
 This app will improve performance of image browsing at the cost of storage space.
 
+## Using [`whiteboard`](https://github.com/nextcloud/whiteboard) app
+
+1. Uncomment the following lines in your env file:
+```
+# COMPOSE_FILE="$COMPOSE_FILE:compose.whiteboard.yml"
+# APPS="$APPS whiteboard"
+# SECRET_WHITEBOARD_JWT_VERSION=v1
+```
+
+2. Generate the secret for whiteboard
+```bash
+abra app secret generate <domain> whiteboard_jwt v1
+```
+make note of this secret for a future step.
+
+3. Deploy your app
+```bash
+abra app deploy -f <domain>
+```
+
+Note: Depending on your deployment, you may need to manually install the Whiteboard app.
+From an admin account, visit
+```
+https://<your domain>/settings/apps/search?q=whiteboard
+```
+and install the "Whiteboard" app.
+
+4. From an admin account, visit the Whiteboard Administration Settings:
+```
+https://<your domain>/settings/admin/whiteboard
+```
+And fill in:
+
+- Websocket URL: `https://<your domain>/whiteboard`
+- Shared Secret: the `whiteboard_jwt` secret you generated earlier
+
 ## Fulltextsearch using elasticsearch
 
 1. Uncomment the following lines in your env file: