Auto merge of #3589 - flip1995:contrib_inactive_prs, r=phansch
Auto merge of #3589 - flip1995:contrib_inactive_prs, r=phansch

Mention S-inactive-closed PRs in the CONTRIBUTING.md

Some of these PRs would be great if they could be completed. But I guess no-one would look through closed PRs if they want to contribute. Let's point (new) contributors there by mentioning these PRs in the `CONTRIBUTING.md`

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust