Remove Atom and add Emacs as a "preferred editor"

Atom is already dead.

But Emacs will outlast human civilization.

Author: jschmidt-icinga

doc/21-development.md

@@ -683,7 +683,7 @@ these tools:
 - vim
 - CLion (macOS, Linux)
 - MS Visual Studio (Windows)
-- Atom
+- Emacs
 
 Editors differ on the functionality. The more helpers you get for C++ development,
 the faster your development workflow will be.