docs: improve contributing guidelines with clearer structure

- Add explanation of why issues are required (maintenance burden)
- Replace label paragraph with table showing who each label is for
- Add guidance to comment on issues before starting work
- Expand PR section with scope guidance and rejection criteria

Author: maxisbey

CONTRIBUTING.md

@@ -6,7 +6,7 @@ Thank you for your interest in contributing to the MCP Python SDK! This document
 
 We welcome contributions! These guidelines exist to save everyone time, yours included. Following them means your work is more likely to be accepted.
 
-**All pull requests require a corresponding issue.** Unless your change is trivial (typo, docs tweak, broken link), create an issue first. PRs without a linked issue will be closed.
+**All pull requests require a corresponding issue.** Unless your change is trivial (typo, docs tweak, broken link), create an issue first. Every merged feature becomes ongoing maintenance, so we need to agree something is worth doing before reviewing code. PRs without a linked issue will be closed.
 
 Having an issue doesn't guarantee acceptance. Wait for maintainer feedback or a `ready for work` label before starting. PRs for issues without buy-in may also be closed.
 
@@ -27,11 +27,17 @@ These always require an issue first:
 
 Bug fixes for clear, reproducible issues are welcome—but still create an issue to track the fix.
 
-### Good Candidates for Contribution
+### Finding Issues to Work On
 
-Issues labeled [`good first issue`](https://github.com/modelcontextprotocol/python-sdk/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22) or [`help wanted`](https://github.com/modelcontextprotocol/python-sdk/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22) are great starting points. Issues labeled [`ready for work`](https://github.com/modelcontextprotocol/python-sdk/issues?q=is%3Aopen+is%3Aissue+label%3A%22ready+for+work%22) have been triaged and are ready for implementation.
+| Label | For | Description |
+|-------|-----|-------------|
+| [`good first issue`](https://github.com/modelcontextprotocol/python-sdk/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22) | Newcomers | Can tackle without deep codebase knowledge |
+| [`help wanted`](https://github.com/modelcontextprotocol/python-sdk/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22) | Experienced contributors | Maintainers probably won't get to this |
+| [`ready for work`](https://github.com/modelcontextprotocol/python-sdk/issues?q=is%3Aopen+is%3Aissue+label%3A%22ready+for+work%22) | Maintainers | Triaged and ready for a maintainer to pick up |
 
-Issues labeled `needs confirmation` or `needs maintainer action` are **not** good candidates—wait for maintainer input before starting work.
+Issues labeled `needs confirmation` or `needs maintainer action` are **not** ready for work—wait for maintainer input first.
+
+Before starting, comment on the issue so we can assign it to you. This prevents duplicate effort.
 
 ## Development Setup
 
@@ -102,13 +108,29 @@ pre-commit run --all-files
 - Add type hints to all functions
 - Include docstrings for public APIs
 
-## Pull Request Process
+## Pull Requests
+
+By the time you open a PR, the "what" and "why" should already be settled in an issue. This keeps reviews focused on implementation.
+
+### Scope
+
+Small PRs get reviewed fast. Large PRs sit in the queue.
+
+A few dozen lines can be reviewed in minutes. Hundreds of lines across many files takes real effort and things slip through. If your change is big, break it into smaller PRs or get alignment from a maintainer first.
+
+### What Gets Rejected
+
+- **No prior discussion**: Features or significant changes without an approved issue
+- **Scope creep**: Changes that go beyond what was discussed
+- **Misalignment**: Even well-implemented features may be rejected if they don't fit the SDK's direction
+- **Overengineering**: Unnecessary complexity for simple problems
+
+### Checklist
 
 1. Update documentation as needed
 2. Add tests for new functionality
 3. Ensure CI passes
-4. Maintainers will review your code
-5. Address review feedback
+4. Address review feedback
 
 ## Code of Conduct