Add readme to hint at mscgen
diff --git a/doc/README.msc b/doc/README.msc
new file mode 100644
index 0000000..580fa8f
--- /dev/null
+++ b/doc/README.msc
@@ -0,0 +1,2 @@
+sudo apt-get install mscgen
+mscgen -T svg hnb_cs_lu.msc