doc: document open pull request limit

Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/65250
Reviewed-By: Aviv Keller <me@aviv.sh>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Stewart X Addison <sxa@redhat.com>
Reviewed-By: Mike McCready <66998419+MikeMcC399@users.noreply.github.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Beth Griggs <bethanyngriggs@gmail.com>

CONTRIBUTING.md

@@ -47,6 +47,8 @@ See [details on our policy on Code of Conduct](./doc/contributing/code-of-conduc
 
 Pull Requests are the way concrete changes are made to the code, documentation,
 dependencies, and tools contained in the `nodejs/node` repository.
+Contributors who are not Collaborators may have no more than 10 pull requests
+open at a time.
 
 * [Dependencies](./doc/contributing/pull-requests.md#dependencies)
 * [Setting up your local environment](./doc/contributing/pull-requests.md#setting-up-your-local-environment)