Add instructions to install redis and include it in Procfile

Procfile

@@ -1,2 +1,3 @@
 web: cd ../zed.dev && PORT=3000 npx next dev
 collab: cd crates/collab && cargo run
+redis: redis-server

README.md

@@ -23,6 +23,12 @@ script/sqlx migrate run
 script/seed-db
 ```
 
+Install Redis:
+
+```
+brew install redis
+```
+
 Run the web frontend and the collaboration server.
 
 ```