Add sponsoring info 💖

.github/FUNDING.yml

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+custom: ['https://github.com/rust-embedded/rust-raspi3-OS-tutorials/blob/master/SPONSORING.md']

SPONSORING.md

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+# 💖 Sponsoring the Developers
+
+This repository is the result of the combined efforts of a large number of developers - in terms of
+writing the source code, but also in performing code reviews, writing documentation, and handling
+issues and PRs. Some of these developers received a salary or other payment from a third-party for
+their contributions. Others receive no payment of any kind, and simply hope that others find their
+contributions useful.
+
+If you would like to support any of the developers, please take a moment to [click through to their
+profiles] to see if they accept sponsorship and/or charitable gifts made in their name.
+
+[click through to their profiles]: https://github.com/rust-embedded/rust-raspi3-OS-tutorials/graphs/contributors