DOC: Add maintainers for the Prometheus exporter

William Dauchy and Christopher Faulet are the official maintainers of the
Prometheus exporter. William better knows the Prometheus usages and is the
referent for the features while Christopher's role is more code-centric, he
takes care of bugs and the exporter integration into Haproxy.

(cherry picked from commit 84c88a58c5153b8fffa509934375c92435298a1b)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/MAINTAINERS b/MAINTAINERS
index 82e8e13..4b00bd7 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -86,3 +86,12 @@
 ScientiaMobile WURFL Device Detection
 Maintainer: Paul Borile, Massimiliano Bellomi <wurfl-haproxy-support@scientiamobile.com>
 Files: src/wurfl.c
+
+Prometheus Exporter
+Maintainer: Christopher Faulet <cfaulet@haproxy.com>
+Maintainer: William Dauchy <wdauchy@gmail.com>
+Files: contrib/prometheus-exporter
+Note: William is the referent regarding Prometheus. He should be consulted for
+      all additions and modifications of features. Christopher is the referent
+      for the code itself. He should be consulted for questions regarding the
+      exporter integration into HAProxy, as well as for the bugs.