Add the freeCodeCamp Guide to Showcase (#2896)

Author: QuincyLarson

README.md

@@ -16,6 +16,7 @@ Websites built with Gatsby:
 * [React](https://reactjs.org/) ([source](https://github.com/reactjs/reactjs.org))
 * [Sourcegraph](https://about.sourcegraph.com)
 * [Simply](https://simply.co.za)
+* [The freeCodeCamp Guide](https://guide.freecodecamp.org)
 * [FloydHub's Blog](https://blog.floydhub.com)
 * [mParticle's Documentation](https://docs.mparticle.com)
 * [Segment's Blog](https://segment.com/blog/)