Add fortitude gem to README (#751)

I learned about the `fortitude` gem in this issue
(#750) and thought I'd also
add it to the prior art section in the README.

README.md

@@ -31,3 +31,4 @@ Docs and more at [Phlex.fun](https://www.phlex.fun/)
 - [compony](https://github.com/kalsan/compony)
 - [tagz](https://github.com/ahoward/tagz)
 - [html](https://github.com/ismasan/html)
+- [fortitude](https://github.com/ageweke/fortitude)