docs: add writing tests section to contributing guide

static/docs/user-guide/contributing/core.md

@@ -25,9 +25,9 @@ paragraphs below to learn how to submit your changes.
   run tests or [run](#running-development-version) the DVC with your changes.
 - Fork [DVC](https://github.com/iterative/dvc.git) and prepare necessary
   changes.
-- Add tests for your changes to `tests/test_*.py`. You can skip this step if the
-  effort to create tests for your change is unreasonable. Changes without tests
-  are still going to be considered by us.
+- [Add tests](#writing-tests) for your changes to `tests/`. You can skip this
+  step if the effort to create tests for your change is unreasonable. Changes
+  without tests are still going to be considered by us.
 - [Run tests](#running-tests) and make sure all of them pass.
 - Submit a pull request, referencing any issues it addresses.
 
@@ -72,6 +72,18 @@ That should be it. You should be ready to make changes, run tests, and make
 commits! If you experience any problems, please don't hesitate to ping us in our
 [chat](/chat).
 
+## Writing tests
+
+We have unit tests and functional ones in `tests/unit/` and `tests/func/`
+correspondingly. Consider writing the former to ensure complicated functions and
+classes behave as expected.
+
+To test a specific functionality you will need to use functional tests alongside
+some [pytest](https://docs.pytest.org/en/latest/) fixtures to create a temporary
+directory, git and/or dvc repo and bootstrap some files. See a
+[dir_helpers module](https://github.com/iterative/dvc/blob/master/tests/dir_helpers.py)
+docstring for some usage examples.
+
 ## Running tests
 
 The simplest way to run tests: