Add Whitespace Warnings as a git til.

git/whitespace-warnings.md

@@ -0,0 +1,13 @@
+# Whitespace Warnings
+
+You can configure git to warn you about whitespace issues in a file when you
+diff it. This is handled by the `core.whitespace` configuration. Add the
+following to your `.gitconfig` file:
+
+```
+[core]
+      whitespace = warn
+```
+
+By default, git will warn you of trailing whitespace at the end of a line as
+well as blank lines at the end of a file.