pythongh-104469: Update README.txt for _testcapi (pythongh-104529)

* pythongh-104469: Update README.txt for _testcapi

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>

Modules/_testcapi/README.txt

@@ -1,3 +1,10 @@
 Tests in this directory are compiled into the _testcapi extension.
 The main file for the extension is Modules/_testcapimodule.c, which
 calls `_PyTestCapi_Init_*` from these functions.
+
+General guideline when writing test code for C API.
+* Use Argument Clinic to minimise the amount of boilerplate code.
+* Add a newline between the argument spec and the docstring.
+* If a test description is needed, make sure the added docstring clearly and succinctly describes purpose of the function.
+* DRY, use the clone feature of Argument Clinic.
+* Try to avoid adding new interned strings; reuse existing parameter names if possible. Use the `as` feature of Argument Clinic to override the C variable name, if needed.