DOC: add Christopher and Emeric as maintainers of the threads

We'll need to be extremely careful at the beginning regarding changes.
diff --git a/MAINTAINERS b/MAINTAINERS
index 35c1043..3a9e435 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -69,3 +69,16 @@
 Master-worker
 Maintainer: William Lallemand <wlallemand@haproxy.com>
 Note: Please CC William when touching src/haproxy.c and contrib/systemd
+
+SPOE
+Maintainer: Christopher Faulet <cfaulet@haproxy.com>
+Files: src/flt_spoe.c, include/*/spoe.h, contrib/spoa_example, doc/SPOE.txt
+
+Multi-threading
+Maintainer: Christopher Faulet <cfaulet@haproxy.com>
+Maintainer: Emeric Brun <ebrun@haproxy.com>
+Files: src/hathreads.c, include/*/hathreads.h
+Note: every change around the locking or synchronization point will require
+      approval from one of these maintainers. Problems which only appear when
+      nbthread is greater than 1 and which disappear otherwise are also
+      relevant.