Edit for readability and fix typos.

Wording changes for easier reading. Fixed typos. Change "meta data" to the more
commonly used "metadata." Changed "Wordpress" to more correct "WordPress."
Fix stray instance of "Wordpress" => "WordPress"
It's "pull request" rather than "pull-request"
Clarify intro to metadata field descriptions
Merge pull request #8 from joshuago/proofread_readme

Proofread README