graphql: document how to add fields
graphql: document how to add fields

closes GQL-39

Test plan:
  * docs make sense

Change-Id: I87e6f7c632acf48eef28d4fb7b535ed67b54eb66
Reviewed-on: https://gerrit.instructure.com/179830
Tested-by: Jenkins
QA-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
Product-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
Reviewed-by: Chris Mena <59127e1605b963f834d84868038adff8ad680e36@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms