docs: add Contributing/Code Formatting section to README

Author: Garrett Moseke

README.md

@@ -1 +1,12 @@
-# swift-numerics-differentiable
+# swift-numerics-differentiable
+
+## Contributing
+### Code Formatting
+This package makes use of [swift-format](https://github.com/swiftlang/swift-format), which is built directly into the swift toolchain as of
+swift 6. To apply formatting rules to all files, which you should do before submitting a PR, run from the root of the repository:
+
+```sh
+swift format --in-place --recursive .
+```
+
+Formatting is validated with the `--strict` flag on every PR