Convert Docusaurus banner image into SVG (#915)

* replaced readme header image

Changed from .PNG to .SVG to eliminate blurriness on high-resolution screens

* compressed header .SVG

* Replaced header image in blog post

Blogpost: Introducing Docusaurus

README.md

@@ -1,6 +1,6 @@
 <h1 align="center">
   <p align="center">Docusaurus</p>
-  <a href="https://docusaurus.io"><img src="/website/static/img/slash-introducing.png" alt="Docusaurus"></a>
+  <a href="https://docusaurus.io"><img src="/website/static/img/slash-introducing.svg" alt="Docusaurus"></a>
 </h1>
 
 <p align="center">

website/blog/2017-12-14-introducing-docusaurus.md

@@ -6,7 +6,7 @@ authorFBID: 611217057
 authorTwitter: JoelMarcey
 ---
 
-![Introducing Slash](/img/slash-introducing.png)
+![Introducing Slash](/img/slash-introducing.svg)
 
 We are very happy to introduce [Docusaurus](https://github.com/facebook/Docusaurus) to help you manage one or many open source websites.