Update rails.md

It occupied me quite a while figuring out why this sample didn't have a restart policy. And why we where supposed to always properly stop the containers wheres you normally would simply hit `Ctrl+C`.

Turned out that this `pid` issue is a rails-specific issue. docker/compose#1393 (comment)

And of course there's already [a Gem for fixing that](https://github.com/lorenzosinisi/shutup) but having the one-liner in the `docker-compose.yml` keeps the logic where it belongs to (and now also is treated here in the sample).

compose/rails.md

@@ -47,11 +47,13 @@ to link them together and expose the web app's port.
     services:
       db:
         image: postgres
+        restart: always
         volumes:
           - ./tmp/db:/var/lib/postgresql/data
       web:
         build: .
-        command: bundle exec rails s -p 3000 -b '0.0.0.0'
+        restart: always
+        command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
         volumes:
           - .:/myapp
         ports: