DOC: mention that Squid correctly responds 400 to PPv2 header

Amos reported that Squid builds 3.5.0.0_20140624 and 3.5.0.0_20140630
were confirmed to respond correctly here and that any version will do
the same.
(cherry picked from commit 9e1382002aa1ba12dcc637870befd077ff887aad)
diff --git a/doc/proxy-protocol.txt b/doc/proxy-protocol.txt
index a2dbcea..a3925a4 100644
--- a/doc/proxy-protocol.txt
+++ b/doc/proxy-protocol.txt
@@ -692,6 +692,7 @@
     - thttpd 2.20c     : 400 Bad Request + abort => pass/optimal
     - mini-httpd-1.19  : 400 Bad Request + abort => pass/optimal
     - haproxy 1.4.21   : 400 Bad Request + abort => pass/optimal
+    - Squid 3          : 400 Bad Request + abort => pass/optimal
   - SSL :
     - stud 0.3.47      : connection abort        => pass/optimal
     - stunnel 4.45     : connection abort        => pass/optimal