Adding documentation on global ignores

README.md

@@ -14,3 +14,16 @@ For more information on gitignore: [gitignore(5)][g5]
 [fk]: http://help.github.com/forking/
 [pr]: http://help.github.com/pull-requests/
 [g5]: http://man.cx/gitignore
+
+## Global Ignores
+
+git has a global configuration that applies rules to all of
+your projects. For example:
+
+    git config --global core.excludesfile=~/.global_ignore
+
+... will apply the rules in ~/.global_ignore for all of your repos.
+
+This is useful if you use an editor (like Emacs) that drops backup files,
+or if you work in an environment that generates binary or intermediate
+files that are always ignored.