add in the Readme how to use meta description tag

In the readme there is no information about how to set the meta description tag.

To use it, you have to set the `description` variable in the template header like this : 

```
---
layout: post
category: api-hours
flickr_set_id:
title: Clermont'ech API Hour &#35;18
description: this will be the meta description tag content
---
```