README.adoc: add a Star History Chart badge/image

Signed-off-by: Jim Klimov <jimklimov+nut@gmail.com>

README.adoc

@@ -217,6 +217,8 @@ which the larger potential sponsors consider when choosing how to help
 FOSS projects. Keeping the lights shining in such a large non-regression
 build matrix is a big undertaking!
 
+image:https://api.star-history.com/svg?repos=networkupstools/nut&type=Date[link="https://star-history.com/#networkupstools/nut&Date" alt="Star History Chart"]
+
 See <<acknowledgements-ci-ops,acknowledgements of organizations which help
 with NUT CI and other daily operations>> for an overview of the shared effort.
 =====