doc: test: mention OS detection preferences in style guideline

Author: theStack

test/functional/README.md

@@ -37,6 +37,10 @@ don't have test cases for.
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
 - Use `f'{x}'` for string formatting in preference to `'{}'.format(x)` or `'%s' % x`.
+- Use `platform.system()` for detecting the running operating system and `os.name` to
+  check whether it's a POSIX system (see also the `skip_if_platform_not_{linux,posix}`
+  methods in the `BitcoinTestFramework` class, which can be used to skip a whole test
+  depending on the platform).
 
 #### Naming guidelines