Add note about needing to be in the `$gdk/gitlab/qa` directory for `bin/qa` to work properly

What does this MR do?

• Add note about needing to be in the $gdk/gitlab/qa directory for bin/qa to work properly

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

I ran into some issues when first starting out, https://gitlab.slack.com/archives/C3JJET4Q6/p1517384135000134

While in $gdk/gitlab, I tried to run the following

qa/bin/qa Test::Instance http://192.168.1.135:3010

An error occurred while loading ./qa/specs/features.
Failure/Error: load file

LoadError:
  cannot load such file -- /Users/eric/Documents/gitlab/gdk-ee/gitlab/qa/specs/features
# ./qa/qa/specs/runner.rb:22:in `perform'
# ./qa/qa/scenario/template.rb:7:in `block in perform'
# ./qa/qa/scenario/template.rb:5:in `tap'
# ./qa/qa/scenario/template.rb:5:in `perform'
# ./qa/qa/scenario/test/instance.rb:22:in `perform'
# ./qa/qa/scenario/template.rb:7:in `block in perform'
# ./qa/qa/scenario/template.rb:5:in `tap'
# ./qa/qa/scenario/template.rb:5:in `perform'
# ./qa/qa/scenario/bootable.rb:14:in `launch!'

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • Not a user-facing change
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?