Document goleak usage
**What**
Expand the Contribution guide so to describe using `goleak` in unit
tests.  This package provides a simple method for detecting goroutine
leaks in tests. This will help ensure that we write safe code.
@@ -117,6 +117,14 @@ If you would like to ammend your commit follow this guide: [Git: Rewriting Histo
Please follow style guide on [this blog post](https://blog.alexellis.io/golang-writing-unit-tests/) from [The Go Programming Language](https://www.amazon.co.uk/Programming-Language-Addison-Wesley-Professional-Computing/dp/0134190440)
+If you are making changes to code that use goroutines, consider adding `goleak` to your test to help ensure that we are not leaking any goroutines. Simply add
+
+```go
+defer goleak.VerifyNoLeaks(t)
+```
+
+at the very beginning of the test, and it will fail the test if it detects goroutines that were opened but never cleaned up at the end of the test.
+
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