Add docs to CONTRIBUTING.md on running workflows locally

Author: simonjbeaumont

CONTRIBUTING.md

@@ -54,13 +54,40 @@ A good patch is:
 3. Documented, adding API documentation as needed to cover new functions and properties.
 4. Accompanied by a great commit message, using our commit message template.
 
-### Run `./scripts/soundness.sh`
+### Run CI checks locally
 
-The scripts directory contains a [soundness.sh script](https://github.com/apple/swift-openapi-generator/blob/main/scripts/soundness.sh) 
-that enforces additional checks, like license headers and formatting style.
+You can run the Github Actions workflows locally using
+[act](https://github.com/nektos/act). To run all the jobs that run on a pull
+request, use the following command:
 
-Please make sure to `./scripts/soundness.sh` before pushing a change upstream, otherwise it is likely the PR validation will fail
-on minor changes such as a missing `self.` or similar formatting issues.
+```
+% act pull_request
+```
+
+To run just a single job, use `workflow_call -j <job>`, and specify the inputs
+the job expects. For example, to run just shellcheck:
+
+```
+% act workflow_call -j soundness --input shell_check_enabled=true
+```
+
+To bind-mount the working directory to the container, rather than a copy, use
+`--bind`. For example, to run just the formatting, and have the results
+reflected in your working directory:
+
+```
+% act --bind workflow_call -j soundness --input format_check_enabled=true
+```
+
+If you'd like `act` to always run with certain flags, these can be be placed in
+an `.actrc` file either in the current working directory or your home
+directory, for example:
+
+```
+--container-architecture=linux/amd64
+--remote-name upstream
+--action-offline-mode
+```
 
 For frequent contributors, we recommend adding the script as a [git pre-push hook](https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks), which you can do via executing the following command in the project root directory: