DOC: add new file intro.txt

This is an introduction to present HAProxy. The aim is to get rid of the
totally obsolete haproxy-en and haproxy-fr files. This file references
another one which is not there yet and which should cover the remaining
part of these obsolete files, which is how to manage the process. The
format is the same as the other docs so it should integrate seamlessly
to existing docs.

README

@@ -344,6 +344,10 @@ in the usual DESTDIR variable.
 
 There is some documentation in the doc/ directory :
 
+    - intro.txt : this is an introduction to haproxy, it explains what it is
+      what it is not. Useful for beginners or to re-discover it when planning
+      for an upgrade.
+
     - architecture.txt : this is the architecture manual. It is quite old and
       does not tell about the nice new features, but it's still a good starting
       point when you know what you want but don't know how to do it.