Added use strict helper script to docs

docs/eslint/global-strict.md

@@ -13,6 +13,32 @@ and should not be an error.
 Strict mode also causes modifications to objects frozen with
 `Object.freeze` to throw TypeErrors, which is desirable.
 
+If you need a quick and easy way to add `'use strict'` to
+every JavaScript file in your project, you can use the
+execute the following shell script at the root of your
+project:
+
+    git ls-files | grep \.js$ |
+    while read f; do
+        if (head -n3 "$f" | grep -Fv 'use strict' > /dev/null); then
+            echo "$f"
+            if (head -n1 "$f" | grep -Fv '#!/' > /dev/null); then
+                sed -i '' -e '1 i\
+                    '\''use strict'\'';
+                ' "$f"
+            else
+                sed -i '' -e '2 i\
+                    '\''use strict'\'';
+                ' "$f"
+            fi
+        fi
+    done
+
+Just save that in a file, `chmod +x` it, and then execute
+it. I recommend you double check that everything worked
+correctly with `git diff` before you lint again and commit
+your changes.
+
 [Official `global-strict` ESLint Rule Documentation][global-strict-docs]
 
 [global-strict-docs]: https://github.com/eslint/eslint/blob/master/docs/rules/global-strict.md