docs(contributing): set expectation that we don't yet have the best guidance

Author: heitorlessa

CONTRIBUTING.md

@@ -96,7 +96,7 @@ We group tests in different categories
 | End-to-end tests  | Gain confidence that a Lambda function with our code operates as expected.                            | It simulates how customers configure, deploy, and run their Lambda function - Event Source configuration, IAM permissions, etc. | Slow (minutes)                                    |
 | Performance tests | Ensure critical operations won't increase latency and costs to customers.                             | CI arbitrary hardware can make it flaky. We'll resume writing perf test after our new Integ/End have significant coverage.      | Fast to moderate (a few seconds to a few minutes) |
 
-Functional tests is mandatory. Maintainers will help indicate whether additional tests are necessary and provide assistance as required.
+**NOTE**: Functional tests are mandatory. We have plans to create a guide on how to create these different tests. Maintainers will help indicate whether additional tests are necessary and provide assistance as required.
 
 ## Finding contributions to work on