docs: add PR limits to contribution guide (#60910)

Add PR limits section explaining:
- 10 open PRs per author cap
- r: too-many-prs label auto-close mechanism
- How to get exception via #clawtributors Discord

Fixes: #38283

CONTRIBUTING.md

@@ -85,6 +85,12 @@ Welcome to the lobster tank! 🦞
 4. **Test/CI-only PRs for known `main` failures** → Don't open a PR. The Maintainer team is already tracking those failures, and PRs that only tweak tests or CI to chase them will be closed unless they are required to validate a new fix.
 5. **Questions** → Discord [#help](https://discord.com/channels/1456350064065904867/1459642797895319552) / [#users-helping-users](https://discord.com/channels/1456350064065904867/1459007081603403828)
 
+## PR Limits
+
+We cap at **10 open PRs per author**. If you exceed this, the `r: too-many-prs` label is added and your PR is auto-closed. This is a hard limit.
+
+For coordinated change sets that genuinely need more than 10 PRs, join the **#clawtributors** channel in Discord and talk to maintainers first.
+
 ## Before You PR
 
 - Test locally with your OpenClaw instance