DOC(README): rule code assignment policy (#72)

The documentation should explain the rule code assignment policy, so
that its clear which code to pick for newly developed rules.

This was my best guess from the existing rules. Is it correct?

README.md

@@ -53,6 +53,16 @@ To enable them, use standard flake8 configuration options for the plugin mode or
 If you encounter a bug or some other problem with TorchFix, please file an issue on
 https://github.com/pytorch-labs/torchfix/issues.
 
+## Rule Code Assignment Policy
+
+New rule codes are assigned incrementally across the following categories:
+
+* **TOR0XX, TOR1XX**: General-purpose `torch` functionality.
+* **TOR2XX**: Domain-specific rules, such as TorchVision.
+* **TOR4XX**: Noisy rules that are disabled by default.
+* **TOR9XX**: Internal rules specific for `pytorch/pytorch` repo, other users should not use these.
+
+TOR0, TOR1 and TOR2 are enabled by default.
 
 ## Rules