add a formatting document

this hopefully helps new users get started with formatting.

didn't want to edit the CONTRIBUTING.md file because it already
has a lot of other stuff in there not specific to formatting.

FORMATTING.md

@@ -0,0 +1,50 @@
+# Formatting Guidelines
+
+All contributions to Xous must comply with two formatting guidelines:
+
+1. Run `rustfmt` using our `rustfmt.toml` with a nightly toolchain
+2. Remove trailing whitespaces
+
+## Resources
+
+Our `rustfmt` requires a Rust nightly toolchain.
+
+Install a nightly toolchain with
+
+`rustup toolchain install nightly`
+
+From here, you may run `rustfmt` on the command line with
+
+`cargo +nightly fmt -p <crate you are editing>`
+
+Unfortunately, one cannot simply run `rustfmt` on a single file with nightly from the command line without changing the default toolchain. However, there are various plugins one can use to help with this.
+
+### vscode
+
+If you are using `vscode`, add these lines to your `settings.json` file:
+
+```json
+"rust-analyzer.rustfmt.extraArgs": [
+        "+nightly"
+    ],
+"files.trimTrailingWhitespace": true,
+"editor.formatOnSave": true,
+```
+
+The default `settings.json` that comes with the repo already has these built in.
+
+### `pre-commit`
+
+[pre-commit](https://pre-commit.com/) is a tool that can be configured to run github actions locally.
+
+The development workflow looks like this:
+
+ 1. the developer installs pre-commit on their machine and runs pre-commit install from within the xous-core directory: it'll set up the necessary dependencies, and sets up a local git pre-commit hook to run them
+ 2. development flows as expected
+ 3. at commit time, one of two things can happen
+    a. code is formatted correctly already, commit gets written
+    b. code is not formatted correctly, commit is aborted, code gets formatted automatically
+
+### Other tools
+
+If you have your own workflow, please contribute a hint to this document so that others can benefit from it!