Prefer to reference git's default configuration, not bespoke

Prefer references to git's default configuration, rather than encouraging customized configurations.

`git config core.excludesFile` defaults to `${XDG_CONFIG_HOME:-~/.config}/git/ignore`. This updates the .gitignore template to suggest the default location for git's global gitignore rather than a customized location.

Author: jasonkarns

railties/lib/rails/generators/rails/app/templates/gitignore.tt

@@ -1,8 +1,8 @@
 # See https://help.github.com/articles/ignoring-files for more about ignoring files.
 #
-# If you find yourself ignoring temporary files generated by your text editor
-# or operating system, you probably want to add a global ignore instead:
-#   git config --global core.excludesfile '~/.gitignore_global'
+# Temporary files generated by your text editor or operating system
+# belong in git's global ignore instead:
+# `$XDG_CONFIG_HOME/git/ignore` or `~/.config/git/ignore`
 
 # Ignore bundler config.
 /.bundle