Add step for including pg in the Gemfile.

When following the instructions step by step, Heroku is broken because
the `pg` gem is not installed. This commit adds a step for doing so in a
logical place and provides a brief introduction to the purpose of the
Gemfile.

lessons/11.yaml

@@ -3,10 +3,26 @@ goal_title: Commit and push to heroku
 goal: |
   
   We just added a whole new feature. Let's push it to heroku so our
-  friends can play with it.
+  friends can play with it. In order to do so, we need to make a
+  small change to our `Gemfile`
 
 steps: |
   
+  Open the file called `Gemfile` in KomodoEdit, or your preferred editor
+  and find the line beginning:
+  
+      gem 'sqlite3'
+  
+  Remove this line and replace it with:
+  
+      group :development do
+        gem 'sqlite3'
+      end
+  
+      group :production do
+        gem 'pg'
+      end
+  
   In your terminal, try: (**note:** there is a period after the word
   add in the first line)
   
@@ -17,6 +33,11 @@ steps: |
 
 explanation: |
   
+  * The Gemfile is a list of all the Ruby libraries your application needs.
+    what we've declared here, is that we want to use the `sqlite3` library
+    while we're developing on our computer (the development group) but when
+    deploying to heroku (the production group) we want to use the `pg` library,
+    which is made for the type of database that Heroku uses.
   * We've done the first three steps before, so we won't go over that again...
   * `heroku rake` executes a rake task on your server, in this case,
     `db:migrate`, just like you did locally.