docs: update contributing

contributing/CODE.md

@@ -36,6 +36,11 @@ Some files that use CPP language extension cannot be formatted as a whole, so in
 - Do not add comments like "wire format encoding" (Encoding class is always wire format) or "check if X" when the function name already says that
 - Assume a competent Haskell reader
 
+**Diff and refactoring:**
+- Avoid unnecessary changes and code movements
+- Never do refactoring unless it substantially reduces cost of solving the current problem, including the cost of refactoring
+- Aim to minimize the code changes - do what is minimally required to solve users' problems
+
 ### Haskell Extensions
 - `StrictData` enabled by default
 - Use STM for safe concurrency

contributing/README.md

@@ -17,6 +17,7 @@ Please discuss the problem you want to solve and your detailed implementation pl
 This files can be used with LLM prompts, e.g. if you use Claude Code you can create CLAUDE.md file in project root importing content from these files:
 
 ```markdown
+@README.md
 @contributing/PROJECT.md
 @contributing/CODE.md
 ```