Update README.markdown

Added information from this forum topic on how to add a bundle to the list: https://groups.google.com/forum/#!searchin/spf13-vim-discuss/install/spf13-vim-discuss/s_PMWxVVogM/otCEQs0ZlsYJ

README.markdown

@@ -213,12 +213,18 @@ spf13-vim contains a curated set of popular vim plugins, colors, snippets and sy
 
 Create `~/.vimrc.bundles.local` for any additional bundles.
 
-To add a new bundle
+To add a new bundle, just add one line for each bundle you want to install. The line should start with the word "Bundle" followed by a string of either the vim.org project name or the githubusername/githubprojectname. For example, the github project [spf13/vim-colors](https://github.com/spf13/vim-colors) can be added with the following command
 
 ```bash
     echo Bundle \'spf13/vim-colors\' >> ~/.vimrc.bundles.local
 ```
 
+Once new plugins are added, they have to be installed.
+
+```bash
+    vim +BundleInstall! +BundleClean +q
+```
+
 ## Removing (disabling) an included plugin
 
 Create `~/.vimrc.local` if it doesn't already exist.