Add format and lint instructions

CONTRIBUTING.md

@@ -5,6 +5,15 @@ conform to the C++ and Python formatting standards define by the CI. The
 checker will run automatically when you push new commits to your branch, and
 will generate a patch you can apply if the checks fail.
 
+To simplify the workflow, you can format and lint locally using
+
+```shell
+lb-format path/to/file
+flake8 path/to/file
+```
+
+Check [Git4LHCb](https://twiki.cern.ch/twiki/bin/view/LHCb/Git4LHCb) for more details.
+
 ## Branches
 
 The `master` branch is used for Run 3 development and data-taking. This is the