HOWTO code formatting (#685)

## Summary
Describing code formatting in the getting started

Author: krugerk

GETTING_STARTED.md

@@ -22,6 +22,30 @@ If it's still not building, or if you find other noteworthy dependencies not lis
 
 Since the Beeminder backend/web application isn't open source (yet), you'll need to ask us to be added to the private repo if you want to be able to point the iOS app at localhost:3000. You can also create a test account/goal on beeminder.com to test against.
 
+#### Code Formatting
+
+The codebase is formatted using swift-format. A check on CI will fail when the code of a meege request does not comply.
+
+##### lane
+
+A lane in fastlane can be used to format the code.
+
+
+##### pre-commit
+The [pre-commit tool](https://pre-commit.com) can be used to configure git with the pre-commit hook provided in the repository.
+
+Install the tool:
+```
+brew install precommit
+pre-commit --install
+```
+
+It will now run at the start of a git commit. 
+It can also be run manually:
+
+`pre-commit run --all-files`
+
+
 #### Contributing
 
 See [Contributing](CONTRIBUTING.md) for more.