Repository: nodejs/node
Pull request: doc: add section stating that very stale PRs should be closed

I just wanted to open this PR to see if there could be some consensus on closing PRs that
have been state for a significant amount of time (I'm proposing 6 months here, this is very
arbitrary and can definitely be changed/generalized).

The fact is that the number of current PRs in the project, as I am opening this one is 492,
although that's not very problematic it feels rather messy to me (without providing any
noticeable value I think?), having a clear policy on when a PR is so stale that it should probably
be closed could be beneficial in trying to clean outdated and stale PRs without the risk to
generate too much controversy (since right now, as far as I can tell, abandoned PRs just stay
there indefinitely and there isn't a clear procedure on what to do with them). 

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Changed files:
filename: doc/contributing/pull-requests.md
patch: @@ -446,6 +446,11 @@ credit for the work they started (either by preserving their name and email
 address) in the commit log, or by using an `Author:` meta-data tag in the
 commit.
 
+If a pull request has been inactive for more than six months, add the `stalled` label
+to it. That will trigger an automation that adds a comment explaining the pull request
+may be close for inactivity, giving a heads-up to the contributor before actually
+closing it if it remains inactive.
+
 ### Approving a change
 
 Any Node.js core collaborator (any GitHub user with commit rights in the