chore(docs): add sponsorship section

Author: ken-matsui

README.md

@@ -21,6 +21,12 @@ See [INSTALL.md](INSTALL.md).
 
 See [GitHub Discussions](https://github.com/orgs/cabinpkg/discussions).
 
+## Sponsorship
+
+This project was initially funded by the [MITOU IT Program](https://www.ipa.go.jp/en/it-talents/mitou/index.html) ([project page (ja)](https://www.ipa.go.jp/jinzai/mitou/it/2018/gaiyou_t-2.html)) — with additional support from [Shigeru Urushibara (漆原 茂) of UL Systems, Inc.](https://www.ulsystems.co.jp/company/management-committee.html).
+
+If you find Cabin useful, please consider [sponsoring me](https://github.com/sponsors/ken-matsui) to support its continued development.
+
 ## Contributing
 
 See [CONTRIBUTING.md](CONTRIBUTING.md).