README: Expand "Writing Unittests" section

This commit clarifies how test cases are expected to import or address
modules of a Sublime Text package as it slightly changed in PR #277.

Author: deathaxe

README.md

@@ -338,17 +338,63 @@ see also: https://docs.python.org/3/library/warnings.html#warning-filter
 UnitTesting is based on python's `unittest` library. 
 Any valid unittest test case is allowed.
 
+Module lookup and imports respect Sublime Text's package ecosystem.
+Global imports lookup modules in:
+
+- `${data}/sublime-text/Lib/python38/`
+- `${data}/sublime-text/Packages/`
+- `${install}/Packages/`
+
+A common ST package's folder structure may look like this:
+
+```
+Packages
+├── ...
+└── Package To Test
+   ├── sub_package
+   |   ├── __init__.py
+   |   └── module.py
+   ├── tests
+   |   ├── __init__.py
+   |   └── test_module.py
+   └── plugin.py
+```
+
+The `Package To Test` is the root package being tested,
+which contains `tests/` as a sub-package, next to vendored python packages 
+or primary business logic to test in `sub_package`.
+
+This differs from how normal python package repositories organize tests next to package's source folder.
+
+It requires package name (`Package To Test`) to be included in all absolute module names.
+
+> [!NOTE]
+>
+> ST packages don't need and should not contain a top-level `__init__.py` module to be treated like a normal python package.
+
+Test modules can use relative imports to access all modules.
+
 Example:
 
-_tests/test_myunit.py_
+_Package To Test/tests/test_module.py_
 
 ```py
+from unittest.mock import patch
 from unittesting import TestCase
 
+from ..sub_package.module import ClassToTest
+
+
 class MyTestCase(TestCase):
 
   def test_something(self):
-    self.assertTrue(True)
+    obj = ClassToTest()
+    self.assertTrue(obj.some_method() == True)
+
+  def test_with_mock(self)
+    # need absolute module name including ST package here
+    with patch("Package To Test.sub_package.module") as mock:
+      ...
 ```