Start dev guidelines (#651)

Author: lubkoll

DEV_GUIDELINES.md

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+# Development Guidelines
+
+## Pull requests
+In order to allow fast and efficient reviews, please create small PRs per independent change instead of big ones for a full feature. 
+
+**Rule of thumb**: It should be easy to formulate the commit message as a one-liner. If that is not the case, then the PR is probably to big.
+
+
+## DoD -- Definition of Done
+
+tbd
+
+
+## Ready for review
+
+tbd