added reference to materialize theme and 'who is using'

README.md

@@ -18,6 +18,9 @@ in the `docs/content` folder of the repository.
 - [j1m.net](https://j1m.net): https://gitlab.com/jwcampbell/j1mnet
 - [vaporsoft.net](http://vaporsoft.net): https://github.com/piedoom/vaporsoft
 - [bharatkalluri.in](https://bharatkalluri.in): https://github.com/BharatKalluri/Blog
+- [verpeteren.nl](http://www.verpeteren.nl)
+- [atlasreports.nl](http://www.atlasreports.nl)
+- [groksome.com](http://www.groksome.com)
 
 ## Contributing
 As the documentation site is automatically built on commits to master, all development

docs/content/documentation/themes/themes-list.md

@@ -6,3 +6,4 @@ weight = 40
 The following themes are available for Gutenberg:
 
 - [Hyde](https://github.com/Keats/gutenberg-hyde)
+- [Materialize](https://github.com/verpeteren/gutenberg-materialize)