Add setup instructions for Rails db

Author: adaam2

README.md

@@ -1,5 +1,32 @@
 # Setup && running the application
 
+Before you run the application for the first time, you must create the database and run migrations:
+
+```
+docker-compose run backend rake db:create db:migrate
+```
+
+Or alternatively, log in via bash and run the commands there:
+```
+docker-compose run backend bash
+root@1c5e8aedd21a:/usr/src/backend-app# rake db:create
+Created database 'backend_development'
+Created database 'backend_test'
+root@1c5e8aedd21a:/usr/src/backend-app# rake db:migrate
+== 20180306161431 DeviseTokenAuthCreateUsers: migrating =======================
+-- create_table(:users)
+ -> 0.0164s
+-- add_index(:users, :email, {:unique=>true})
+ -> 0.0088s
+-- add_index(:users, [:uid, :provider], {:unique=>true})
+ -> 0.0078s
+-- add_index(:users, :reset_password_token, {:unique=>true})
+ -> 0.0092s
+-- add_index(:users, :confirmation_token, {:unique=>true})
+ -> 0.0070s
+== 20180306161431 DeviseTokenAuthCreateUsers: migrated (0.0497s) ==============
+```
+
 Running the app (and navigate to `http://localhost:8080`):
 
 ```