Add Rakefile and update README.

Author: davidwinter

README.md

@@ -3,3 +3,9 @@ You can use this as a start point for managing a WordPress install with git.
 In it's current state, all you need to do is clone this repo, `git submodule update --init` then copy `wp-config-sample.php` to `wp-config.php`, update your database settings, and your default theme.
 
 See '[Install and manage WordPress with Git](http://davidwinter.me/articles/2012/04/09/install-and-manage-wordpress-with-git/)' for more details.
+
+## Updating WordPress
+
+You can use the Rake helper task to upgrade WordPress to a specific version:
+
+	rake wordpress_update[3.7.1]

Rakefile

@@ -0,0 +1,28 @@
+task :default => [:install]
+
+desc 'Setup subrepository'
+task :install do
+  system 'git submodule update --init'
+end
+
+# Usage:
+#
+#     rake wordpress_update[3.7.1]
+#
+desc 'Update Wordpress'
+task :wordpress_update, :version do |t, args|
+  puts args[:version]
+  commands = [
+    'git pull',
+    'cd wordpress',
+    'git checkout master',
+    'git fetch --tags',
+    "git checkout #{args[:version]}",
+    'cd ..',
+    'git add wordpress',
+    "git commit -m 'Update Wordpress to #{args[:version]}'",
+    'git push',
+  ]
+
+  system commands.join(' && ')
+end