DEV: haring: update readme to suggest using the same build options fo…

…r haring

It's not necessarily obvious so better suggest it there to use the same
build options, and indicate the tradeoffs (e.g. depend on more libs).

dev/haring/README

@@ -2,3 +2,8 @@ This needs to be built from the top makefile, for example :
 
   make dev/haring/haring
 
+If HAProxy is built with special options such -DDEBUG_THREAD or with
+multi-threading support enabled (which changes the ring's header size),
+it can be worth reusing the same build options for haring, usually they
+will remain compatible, and will simplify the handling of different file
+layouts, at the expense of dragging more dependencies into the executable.