Clearer documentation/examples for ordering by meta keys

I found it quite confusing when trying to sort a mini loop using the values of a postmeta custom field.

This PR updates the documentation to explain how to order by a custom field value, either using alphabetical sorting or numeric sorting.

See http://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters for more details.

You can see what this looks like by going to http://wordpress.org/plugins/about/validator/ and entering `https://raw.github.com/om4james/mini-loops/orderbymeta/readme.txt` in the readme URL field.