Update README.md

Minor tweaks to readme

Author: tkadlec

README.md

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-![WebPageTest GitHub Action](actions-card.png)
+<p align="center"><img src="https://docs.webpagetest.org/img/wpt-navy-logo.png" alt="WebPageTest Logo" /></p>
+<h1 align="center">WebPageTest GitHub Action</h1>
 
-# WebPageTest Github Action
 WebPageTest's GitHub Action lets you automatically run tests against WebPageTest on code changes. You can set and enforce performance budgets, and have performance data automatically added to your pull requets to move the performance conversation directly into your existing development workflow.
 
 
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