Add gumroad link

manhtai · manhtai · commit 12eeec5906df · 2021-10-02T20:27:35.000+07:00
diff --git a/README.md b/README.md
@@ -24,6 +24,10 @@ For macOS & Windows: https://github.com/plainlab/plainprinter/releases
 
 https://github.com/manhtai/vitalsource-printer
 
+## Got a dollar or two?
+
+Buy it: https://gum.co/plainprinter
+
 ---
 
 &copy; 2021 PlainLab
