Mention writeas-gtk is libre, not gratis

README.md

@@ -5,6 +5,8 @@ Android. It lets you compose and publish posts to [Write.as](https://write.as/).
 
 For a UI toolkit it uses GTK, and relies on the [writeas-cli](https://github.com/writeas/writeas-cli) for API calls and post management.
 
+**Write.as GTK is _libre_, not _gratis_, software.** Please try the app free of charge, but [purchase a copy](https://write.as/apps/linux/get) if you continue using it.
+
 ## Usage
 
 See the [User Guide](https://code.as/writeas/writeas-gtk/src/branch/master/USER_GUIDE.md).