Add co-maintainership invitation

Author: cyqsimon

README.md

@@ -37,6 +37,8 @@ This is a CLI utility for displaying current network utilization by process, con
 This project is in passive maintenance. Critical issues will be addressed, but
 no new features are being worked on. However, this is due to a lack of funding
 and/or manpower more than anything else, so pull requests are more than welcome.
+In addition, if you are able and willing to contribute to this project long-term,
+we would like to invite you to apply for co-maintainership.
 
 For more details, see [The Future of Bandwhich #275](https://github.com/imsnif/bandwhich/issues/275).