Update CONTRIBUTING.md (elastic#80106)

* Update CONTRIBUTING.md

Manual steps in the Formatting section to setup ECF did not work for me.

Added alternative steps to import build-conventions/formatterConfig.xml
into IntelliJ 2021.2.3 Ultimate on Windows.

Author: justincr-elastic

CONTRIBUTING.md

@@ -196,6 +196,13 @@ need them.
    5. **IMPORTANT** - make sure "Optimize Imports" is **NOT** selected.
    6. Click "OK"
 
+Alternative manual steps for IntelliJ.
+
+   1. Open **File > Settings/Preferences > Code Style > Java**
+   2. Gear icon > Import Scheme > Eclipse XML Profile
+   3. Navigate to the file `build-conventions/formatterConfig.xml`
+   4. Click "OK"
+
 Note that only some sub-projects in the Elasticsearch project are currently
 fully-formatted. You can see a list of project that **are not**
 automatically formatted in