Add Ed McFarlane as a maintainer (#680)

This proposes adding @emcfarlane (from Buf) to the list of maintainers.
Per [Connect's governance
policy](https://connectrpc.com/docs/governance/project-governance), this
also makes Ed one of the handful of maintainers with a vote on Connect
RFCs.

Ed has done the lion's share of recent maintenance and improvements in
[this
codebase](https://github.com/connectrpc/connect-go/pulls?q=is%3Apr+is%3Aclosed+author%3Aemcfarlane)
(as well as in
[otelconnect](https://github.com/connectrpc/otelconnect-go/pulls?q=is%3Apr+is%3Aclosed+author%3Aemcfarlane)),
and he is already a maintainer in several related packages
([authn](https://github.com/connectrpc/authn-go/blob/main/MAINTAINERS.md),
[cors](https://github.com/connectrpc/cors-go/blob/main/MAINTAINERS.md),
and
[vanguard](https://github.com/connectrpc/vanguard-go/blob/main/MAINTAINERS.md)).
He is an expert in all three of the protocols (as well as in RPC<-> REST
transcoding), has made numerous high-quality contributions to the code,
and is already a valuable resource for answering questions in Slack and
in GitHub issues.

I am in support of this. @akshayjshah, @bufdev, @mattrobenolt, what do
you think?

MAINTAINERS.md

@@ -6,6 +6,7 @@ Maintainers
 * [Akshay Shah](https://github.com/akshayjshah), [Buf](https://buf.build)
 * [Josh Humphries](https://github.com/jhump), [Buf](https://buf.build)
 * [Matt Robenolt](https://github.com/mattrobenolt), [PlanetScale](https://planetscale.com)
+* [Edward McFarlane](https://github.com/emcfarlane), [Buf](https://buf.build)
 
 ## Former
 * [Alex McKinney](https://github.com/amckinney)