Added more to code style

docs/developer/code-style.md

 # Developer - Code style
 
 - Variable names should alway be in *camelcase* and do **not** start with a capital letter
-- Class names should alway be in *Camelcase* and do **always** start with a capital letter
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+- Class names should alway be in *Camelcase* and do **always** start with a capital letter
+- Avoid using null, the Option type in scala can be used instead
+- If a method/value is designed to be overridden make it a `def` and override it with a `def`, do not use `val`
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