Document diffing directories

Also add sample directories to exercise this.

Helps with #98.

manual/src/getting_started.md

@@ -20,3 +20,12 @@ $ difft sample_files/before.js sample_files/after.js
 ```
 
 Difftastic uses the file extension to decide which parser to use.
+
+## Diffing Directories
+
+```
+$ difft sample_files/dir_before sample_files/dir_after
+```
+
+Difftastic will recursively walk the two directories, diffing files
+with the same name.

sample_files/dir_after/foo.js

@@ -0,0 +1,9 @@
+// hello
+if (true) {
+  foo();
+  bar(1);
+  baz();
+}
+
+var people = ["john", "harry", "dick", "yvonne",
+              "archibald", "jenny", "alexandra"];

sample_files/dir_before/foo.js

@@ -0,0 +1,7 @@
+// hello
+foo();
+bar();
+baz();
+
+var people = ["john", "harry", "dick", "archibald",
+              "jenny", "alexandra"];