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).

Author: Norman Kämper-Leymann

compose/rails.md

@@ -77,6 +77,7 @@ restart the containers.
     services:
       db:
         image: postgres
+        restart: always
         volumes:
           - ./tmp/db:/var/lib/postgresql/data
       web:
@@ -256,18 +257,6 @@ Removing rails_db_1 ... done
 Removing network rails_default
 ```
 
-You can also stop the application with `Ctrl-C` in the same shell in which you
-executed `docker-compose up`.  If you stop the app this way, and attempt to
-restart it, you might get the following error:
-
-```none
-web_1 | A server is already
-running. Check /myapp/tmp/pids/server.pid.
-```
-
-To resolve this, delete the file `tmp/pids/server.pid`, and then restart the
-application with `docker-compose up`.
-
 ### Restart the application
 
 To restart the application run `docker-compose up` in the project directory.