Add a note in CONTRIBUTING on how to substitute with a local Physis

CONTRIBUTING.md

@@ -42,3 +42,18 @@ Before making a pull request, make sure:
 * Kawari compiles and runs fine. At a minimum, you should be able to login to the World server.
 * Run `cargo fmt` to ensure your code is formatted.
 * Run `cargo clippy` and fix all of the warnings for any new code, to the best of your ability.
+
+## Testing local Physis
+
+Sometimes, you need to be able to test changes without committing or
+pushing changes to Physis. But since we depend on other crates that define
+their dependency on Physis you need to do this a certain way.
+
+Add this line to the bottom of your `Cargo.toml`:
+
+```toml
+[patch."https://github.com/redstrate/Physis"]
+physis = { path = "/path/to/your/Physis" }
+```
+
+This will ensure all of the dependencies target your local Physis checkout and you don't end up with multiple conflicting library versions.