docs: Document strategy for incremental adoption of static typing (me…

…ltano#6716)

docs/src/_contribute/style.md

@@ -22,9 +22,6 @@ Python:
 Flake8 is a python tool that glues together `pycodestyle`, `pyflakes`, `mccabe`, and third-party plugins to check the style and quality of python code,
 and `wemake-python-styleguide` is a plugin for Flake8 that offers an extensive set of opinionated rules that encourage clean and correct code.
 
-[MyPy is being adopted incrementally by this codebase](https://github.com/meltano/meltano/issues/6491). If you are adding new code, please use type hints.
-To enable type checks for an existing module, _de-glob_ it in the whitelist in `pyproject.toml` and fix any errors.
-
 Javascript:
 
 - [ESLint](https://eslint.org/docs/rules/)
@@ -33,6 +30,43 @@ Javascript:
 
 You may use `make lint` to automatically lint all your code, or `make show_lint` if you only want to see what needs to change.
 
+### Static typing
+
+[MyPy is being adopted incrementally by this codebase](https://github.com/meltano/meltano/issues/6715). If you are adding new code, please use type hints.
+
+If you are making changes to existing Python modules, you are encouraged to enable type checks. To do so,
+
+- if you are fixing a single `.py` file inside a sub-package, e.g. `meltano.core.plugin.command`, _de-glob_ the sub-package in `pyproject.toml` and enumerate the files you are not touching. This will allow you to run `nox -rs mypy` and catch type errors only in the files you are touching.
+
+  ```diff
+    [[tool.mypy.overrides]]
+    module = [
+      ...
+      "meltano.core.m5o.*",
+  -   "meltano.core.plugin.*",
+  +   "meltano.core.plugin.airflow",
+  +   "meltano.core.plugin.base",
+  +   # Note that meltano.core.plugin.command is not included here
+  +   "meltano.core.plugin.config_service",
+  +   "meltano.core.plugin.dbt.*",
+  +   "meltano.core.plugin.error",
+  +   "meltano.core.plugin.factory",
+  +   "meltano.core.plugin.file",
+  +   "meltano.core.plugin.meltano_file",
+  +   "meltano.core.plugin.model.*",
+  +   "meltano.core.plugin.project_plugin",
+  +   "meltano.core.plugin.requirements",
+  +   "meltano.core.plugin.settings_service",
+  +   "meltano.core.plugin.singer.*",
+  +   "meltano.core.plugin.superset",
+  +   "meltano.core.plugin.utility",
+      "meltano.core.runner.*",
+      ...
+    ]
+  ```
+
+- if you are fixing an entire sub-package, simply remove it from the ignore list.
+
 ### Mantra
 
 > A contributor should know the exact line-of-code to make a change based on convention