DOC: mention management.txt in README

and in intro.txt as well. Remove references to haproxy-{en,fr}.txt.
diff --git a/doc/intro.txt b/doc/intro.txt
index 19b4b1e..05d3e66 100644
--- a/doc/intro.txt
+++ b/doc/intro.txt
@@ -78,6 +78,10 @@
     ensure you're running with all known fixes how to update it, complements and
     alternatives.
 
+  - management.txt : it explains how to start haproxy, how to manage it at
+    runtime, how to manage it on multiple nodes, how to proceed with seamless
+    upgrades.
+
   - configuration.txt : the reference manual details all configuration keywords
     and their options. It is used when a configuration change is needed.