Add developer notes about PR reviewing (#157)

Author: mikevoronov

docs/developer-notes.md

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+In the Marine we use the following conventions for PR reviewing:  
+- **Concept ACK** - agree with the idea and overall concept, but haven't reviewed the code changes or tested them,
+- **utACK (untested ACK)** - reviewed and agree with the code changes but haven't actually tested them,
+- **Tested ACK** - reviewed the code changes and have verified the functionality or a bug fix, 
+- **NACK** - disagree with the code changes/concept, should be accompanied by an explanation.