Add docs for new 'gems' option.

site/docs/configuration.md

@@ -281,6 +281,7 @@ layouts:     ./_layouts
 include:     ['.htaccess']
 exclude:     []
 keep_files:  ['.git','.svn']
+gems:        []
 timezone:    nil
 encoding:    nil
 

site/docs/plugins.md

@@ -25,9 +25,27 @@ having to modify the Jekyll source itself.
 
 ## Installing a plugin
 
-In your site source root, make a `_plugins` directory. Place your plugins here.
-Any file ending in `*.rb` inside this directory will be loaded before Jekyll
-generates your site.
+You have 2 options for installing plugins:
+
+1. In your site source root, make a `_plugins` directory. Place your plugins here.
+    Any file ending in `*.rb` inside this directory will be loaded before Jekyll
+    generates your site.
+2. In your `_config.yml` file, add a new array with the key `gems` and the values
+    of the gem names of the plugins you'd like to use. An example:
+
+        gems: [jekyll-test-plugin, jekyll-jsonify, jekyll-assets]
+        # This will require each of these gems automatically.
+
+<div class="note info">
+  <h5>
+    <code>_plugins</code> and <code>gems</code>
+    can be used simultaneously
+  </h5>
+  <p>
+    You may use both of the aforementioned plugin options simultaneously in the
+    same site if you so choose. Use of one does not restrict the use of the other
+  </p>
+</div>
 
 In general, plugins you make will fall into one of three categories: