chore: add Gui as a maintainer

Pull-Request: #5538.

Author: guillaumemichel

README.md

@@ -78,6 +78,7 @@ Conduct](https://github.com/ipfs/community/blob/master/code-of-conduct.md).
 
 (In alphabetical order.)
 
+- Guillaume Michel ([@guillaumemichel](https://github.com/guillaumemichel))
 - João Oliveira ([@jxs](https://github.com/jxs))
 
 ## Notable users