docs: add TokenTip tip badge to the README

Author: copyleftdev

README.md

@@ -6,6 +6,8 @@
 |_| |_|\___/ \___/|_|\_\_.__/|_|_| |_|
 ```
 
+[![Tip my tokens](https://tokentip.to/badge/copyleftdev.svg?logo=1)](https://tokentip.to/@copyleftdev)
+
 **Single-binary webhook inbox. Accept HTTP, store it, show it. No external dependencies.**
 
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