Simplify .gitignore

This removes the cruft that's grown over time in the .gitignore file with a message about how to create global .gitignore files.

As discussed in #879

Author: leebyron

.gitignore

@@ -1,12 +1,10 @@
-*.swp
-*~
-*.iml
-.*.haste_cache.*
-.DS_Store
-.idea
-npm-debug.log
+# This .gitignore only ignores files specific to this repository.
+# If you see other files generated by your OS or tools you use, consider
+# creating a global .gitignore file.
+#
+# https://help.github.com/articles/ignoring-files/#create-a-global-gitignore
+# https://www.gitignore.io/
 
 node_modules
 coverage
 dist
-.vscode