add contract testing readme
add contract testing readme

refs QUIZ-4175

Test Plan:
  - qa-cr

Change-Id: I8713c0dd007000037e774136ee37770eb90c9970
Reviewed-on: https://gerrit.instructure.com/149118
Tested-by: Jenkins
Reviewed-by: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>
Product-Review: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>
QA-Review: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>

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