Add mention of formatter plugin to CONTRIBUTING.md (#10853)

.github/CONTRIBUTING.md

@@ -14,6 +14,8 @@ Unless the issue is specific to a single version of the Framework, you should al
 
 In your repo, make a new branch from `master`. Make your code changes in that branch, and once you're done make a pull request to the `vaadin/framework` branch `master`.
 
+The `vaadin/framework` project uses `formatter-maven-plugin` to format the code. It will be executed as part of the Maven build. Make sure to run `mvn compile` before making the commit.
+
 We recommend making sure that the "Allow edits from maintainers" checkbox on the pull request page is checked. This allows us to commit minor fixes, like correcting typos, without bothering you.
 
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