Add Philip Warren as a maintainer (#87)

@pkwarren has been working as a primary contributor to connectrpc
libraries. He triages and fixes issues, and is a core collaborator. I'm
proposing he is made a maintainer.

Signed-off-by: Edward McFarlane <3036610+emcfarlane@users.noreply.github.com>

Author: emcfarlane

MAINTAINERS.md

@@ -5,6 +5,7 @@ Maintainers
 * [Josh Humphries](https://github.com/jhump), [Buf](https://buf.build)
 * [Edward McFarlane](https://github.com/emcfarlane), [Buf](https://buf.build)
 * [Timo Stamm](https://github.com/timostamm), [Buf](https://buf.build)
+* [Philip Warren](https://github.com/pkwarren), [Buf](https://buf.build)
 
 ## Former
 * [Akshay Shah](https://github.com/akshayjshah)