explain what we want and what not in .gitignore

Author: RalfJung

.gitignore

@@ -1,3 +1,7 @@
+# This file should only ignore things that are generatd during a build,
+# generated by common IDEs, and optional files controlled by the user
+# that affect the build (such as config.toml).
+# FIXME: This needs cleanup.
 *~
 .#*
 .DS_Store