docs: describe commandDir() parameters (yargs#1540)

docs/api.md

@@ -193,6 +193,39 @@ var argv = require('yargs')
   .argv
 ```
 
+<a name="commandDir"></a>
+.commandDir(directory, [opts])
+------------------------------
+
+Apply command modules from a directory relative to the module calling this method.
+
+`directory` is a relative directory path as a string (required).
+
+`opts` is an options object (optional). The following options are valid:
+
+- `recurse`: boolean, default `false`
+
+    Look for command modules in all subdirectories and apply them as a flattened
+    (non-hierarchical) list.
+
+- `extensions`: array of strings, default `['js']`
+
+    The types of files to look for when requiring command modules.
+
+- `visit`: function
+
+    A synchronous function called for each command module encountered. Accepts
+    `commandObject`, `pathToFile`, and `filename` as arguments. Returns
+    `commandObject` to include the command; any falsy value to exclude/skip it.
+
+- `include`: RegExp or function
+
+    Whitelist certain modules. See [`require-directory` whitelisting](https://www.npmjs.com/package/require-directory#whitelisting) for details.
+
+- `exclude`: RegExp or function
+
+    Blacklist certain modules. See [`require-directory` blacklisting](https://www.npmjs.com/package/require-directory#blacklisting) for details.
+
 <a name="command"></a>
 .command(cmd, desc, [builder], [handler])
 -----------------------------------------