Adding where does test go section in writing-test.md

doc/guides/writing-tests.md

@@ -18,6 +18,15 @@ Add tests when:
 - Fixing regressions and bugs.
 - Expanding test coverage.
 
+## Test directory structure
+See the test [directory structure overview](https://github.com/nodejs/node/blob/master/test/README.md)
+for an outline of existing test types and their locations.
+When deciding on whether to expand an existing test file or create a new one, 
+consider going through the files related to the subsystem.
+For example, take a look at tests starting with "test-streams" 
+when writing tests for `lib/streams.js`
+
+
 ## Test structure
 
 Let's analyze this basic test from the Node.js test suite: