Add link to Jekyll Project Version Tag plugin

Hi, 
I have added a link to my tag plugin that grabs a version identifier from the `git` repo so that you can display what version of site is deployed; hopefully it is useful to other people.

This plugin will look for the latest tag via `git describe --tags` falling back to `git rev-parse HEAD` if there are no tags.

The following snippet will output something like the html below;

```ruby
<p>Build: {% project_version %}</p>
```

```html
<p>Build: 3.0.0-5-ga189420</p>
```