docs: Document environment prep before local development (#1975)

Author: ashvinnihalani

README.md

@@ -49,6 +49,14 @@ Observability guide: [docs/observability.md](./docs/observability.md)
 
 ## If you REALLY want to contribute still.... read this first
 
+Before local development, prepare the environment and install dependencies:
+
+```bash
+# Optional: only needed if you use mise for dev tool management.
+mise install
+bun install .
+```
+
 Read [CONTRIBUTING.md](./CONTRIBUTING.md) before opening an issue or PR.
 
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