Add our statement on code quality

Author: koppor

docs/code-quality.md

@@ -11,3 +11,10 @@ We strongly recommend to read following two books on code quality:
 * [Java by Comparison](https://github.com/JabRef/jabref/tree/c81740b3818c7f9311a6d7ff063243e672c821b4/docs/java.by-comparison.com) is a book by three JabRef developers which focuses on code improvements close to single statements. It is fast to read and one gains much information from each recommendation discussed in the book.
 * [Effective Java](https://www.oreilly.com/library/view/effective-java-3rd/9780134686097/) is the standard book for advanced Java programming. Did you know that `enum` is the [recommended way to enforce a singleton instance of a class](https://learning.oreilly.com/library/view/effective-java-3rd/9780134686097/ch2.xhtml#lev3)? Did you know that one should [refer to objects by their interfaces](https://learning.oreilly.com/library/view/effective-java-3rd/9780134686097/ch9.xhtml#lev64)?
 
+This is how we ensure code quality in JabRef:
+
+* we review each external pull request by at least two [JabRef Core Developers](https://github.com/JabRef/jabref/blob/master/DEVELOPERS).
+* we document our design decisions using the lightweight architectural decision records [MADR](https://adr.github.io/madr/).
+* we follow the principles of [Java by Comparison](https://java.by-comparison.com/).
+* we follow the principles of [Effective Java](https://www.oreilly.com/library/view/effective-java-3rd/9780134686097/).
+* we use [Design Pattners](https://java-design-patterns.com/patterns/) when applicable.