Installation from source guide missing instructions for pg_trgm extension creation

Walking through the installation from source guide the following error was thrown when running the setup rake task:

/home/git/gitlab# sudo -u git -H bundle exec rake gitlab:setup RAILS_ENV=production
This will create the necessary database tables and seed the database.
You will lose any previous data stored in the database.
Do you want to continue (yes/no)? yes

-- enable_extension("plpgsql")
   -> 0.0124s
-- enable_extension("pg_trgm")
rake aborted!
ActiveRecord::StatementInvalid: PG::UndefinedFile: ERROR:  could not open extension control file "/usr/share/postgresql/9.3/extension/pg_trgm.control": No such file or directory
: CREATE EXTENSION IF NOT EXISTS "pg_trgm"

This was rectified by installing the postgresql-contrib package and creating the pg_trgm extension in the production database before continuing with the migration task as per these instructions.

sudo apt-get install postgresql-contrib
sudo -u postgres psql -d gitlabhq_production
CREATE EXTENSION pg_trgm;
\q
sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production