Clarify that the Terraform testing documentation should be read befor…

…e our acceptance testing docs

contributing/writing-tests.md

@@ -1,9 +1,10 @@
 # Writing Acceptance Tests
 
 Terraform includes an acceptance test harness that does most of the repetitive
-work involved in testing a resource. For additional information about testing
-Terraform Providers, see the [Extending Terraform documentation](https://www.terraform.io/docs/extend/testing/index.html).
-This guide is meant to augment that documentation with information specific to the HCP provider.
+work involved in testing a resource. Please read the [Extending Terraform documentation](https://www.terraform.io/docs/extend/testing/index.html)
+first for a general understanding of how this test harness works. The guide
+below is meant to augment that documentation with information specific to the
+HCP provider.
 
 ## Acceptance Tests Often Cost Money to Run