DOC: spoe: Add a note about fragmentation support in HAProxy

Add a note in SPOE.txt to make it clear that HAPRoxy does not support the
fragmentation. It can send fragmented frames if an agent supports it but it
cannot receives and handles fragmented frames.

This patch should fix the issue #659. It may be backported as far as 1.8.
diff --git a/doc/SPOE.txt b/doc/SPOE.txt
index dfc0b71..71c7fde 100644
--- a/doc/SPOE.txt
+++ b/doc/SPOE.txt
@@ -761,6 +761,10 @@
 
 Unsupported or unknown capabilities are silently ignored, when possible.
 
+NOTE: HAProxy does not support the fragmentation for now. This means it is not
+      able to handle fragmented frames. However, if an agent announces the
+      fragmentation support, HAProxy may choose to send fragemented frames.
+
 3.2.2. Frame types overview
 ----------------------------