DOC: REGTESTS README varnishtest -Dno-htx= define.

Add -Dno-htx= to varnishtest command line, because without no-htx most
vtc tests fail (${no-htx} macro). Also link to doc/regression-testing.txt
diff --git a/reg-tests/README b/reg-tests/README
index 5427ad6..05d7e54 100644
--- a/reg-tests/README
+++ b/reg-tests/README
@@ -8,7 +8,7 @@
 developed to test Varnish cache application. varnishtest has been modified in
 collaboration with Varnish cache conceptor Poul-Henning Kamp to support HAProxy in
 addition to Varnish cache.
-
+See also: doc/regression-testing.txt
 
 * varnishtest compilation *
 
@@ -35,7 +35,7 @@
   You must set HAPROXY_PROGRAM environment variable to give the location
   of the HAProxy program to test to varnishtest:
 
-    $ HAPROXY_PROGRAM=<my haproxy program> varnishtest ...
+    $ HAPROXY_PROGRAM=<my haproxy program> varnishtest [-Dno-htx=] ...
 
   The HAProxy VTC files found in HAProxy sources may be run with the reg-tests
   Makefile target. You must set the VARNISHTEST_PROGRAM environment variable to