note sys.platform requirements in contributing

Contributing.md

@@ -228,6 +228,7 @@ handy if you want to step through the testing code to be sure of anything.
 Otherwise, see the [pytest documentation](https://docs.pytest.org/en/stable/contents.html). 
 
 ---
+
 ## Debugging network sends
 
 Rather than risk sending bad data to a remote service, even if only through
@@ -484,6 +485,20 @@ Please be verbose here, more info about weird choices is always prefered over ma
 
 Additionally, if your hack is over around 5 lines, please include a `# HACK END` or similar comment to indicate the end of the hack.
 
+# Use `sys.platform` for platform guards
+
+`mypy` (and `pylance`) understand platform guards and will show unreachable code / resolve imports correctly
+for platform specific things. However, this only works if you directly reference `sys.platform`, importantly 
+the following does not work:
+
+```py
+from sys import platform
+if platform == 'darwin':
+  ...
+```
+
+It **MUST** be `if sys.platform`.
+
 ---
 
 ## Build process