Docs: Make the example more obviously an example (#80)

In a recent support ticket, a user had copied the example `:repo:` line into their `Aptfile`,
which previously referred to the now obsolete "artful" Ubuntu LTS release.

This caused the build to fail after #79, when previously the error was ignored.

This PR makes the example more clearly just an example, and emphasises that
using a custom repository is only necessary if using packages from outside
the standard Ubuntu repository.

In addition, the bogus `pg` Gem example was removed, since `libpq-dev` is
part of the stack image, so we should not be encouraging people to install
it unnecessarily.

README.md

@@ -8,7 +8,13 @@ Added ability to also specify custom repositories through **:repo:** in `Aptfile
 
 This buildpack is not meant to be used on its own, and instead should be in used in combination with Scalingo's [multiple buildpack support](https://doc.scalingo.com/buildpacks/multi).
 
-Include a list of apt package names to be installed in a file named `Aptfile`
+Include a list of apt package names to be installed in a file named `Aptfile`.
+
+To find out what packages are available, see:
+<https://packages.ubuntu.com>
+
+See the [Heroku Stacks](https://devcenter.heroku.com/articles/stack) documention for which
+Ubuntu LTS version is used by each Heroku stack.
 
 #### Setup
 
@@ -32,10 +38,12 @@ Note that the order of the buildpacks in the `.buildpacks` file matters.
 #### Aptfile
 
     # you can list packages
-    libpq-dev
+    libexample-dev
+
     # or include links to specific .deb files
     http://downloads.sourceforge.net/project/wkhtmltopdf/0.12.1/wkhtmltox-0.12.1_linux-precise-amd64.deb
-    # or add custom apt repos
+
+    # or add custom apt repos (only required if using packages outside of the standard Ubuntu APT repositories)
     :repo:deb http://cz.archive.ubuntu.com/ubuntu artful main universe
 
 #### Gemfile