[Docs] Create CONTRIBUTING.md with build instructions (closes #3)

[shuaitao105]

Summary

[$30 BOUNTY] [Docs] Create CONTRIBUTING.md with build instructions

Closes #3

Changes

Recent commits on bounty-3-30-bounty-docs-create-c:

d8b60b8 fix: poll for encryptly .logd to avoid preflight false failures
7b40fce fix: UTF-8 stdio for build.py on Windows GBK consoles
a61a11f Add build diagnostics for 9e38483d
9e38483 Fix UnicodeEncodeError on Windows GBK consoles in build.py
92bf166 Add build diagnostics for 94e2eddd
94e2edd Add CONTRIBUTING.md with setup, build, and PR guidelines
db99170 chore: macos fix

Testing

• Local build passed (dashboard review pipeline)
• Checklist passed

Checklist

• Relevant modules build locally
• Tests pass locally
• Changes scoped to bounty purpose