readme update request: install instructions

[austinhutchison]

I'm leaving a note that the the plugin assumes your layout has a placeholder named meta which needs to come before the component, and this isn't straightforward in the existing documentation.

{% component 'SeoBlogPost' data=post %}    
{% placeholder meta %} 

I'm new to OctoberCMS, so I could be missing something - but the install was a bit of a headache for me.