Add a hint on new document

NGSPICE on Red Hat Like Distributions.pdf

INSTALL

@@ -18,6 +18,7 @@ Table of contents
     1.5.1 Most useful options
     1.5.2 Options Specific to Enable Ngspice as a shared library
     1.5.3 Options Useful for Debugging Ngspice
+   1.6 Installation on Red Hat, Oracle or Centos Linux
   2 Compilers and Options
   3 Compiling For Multiple Architectures
   4 Installation Names
@@ -307,6 +308,14 @@ This file describes the procedures to install ngspice from sources.
  problem yourself, then the development team will love to hear from you.
 
 
+1.6 Installation on Red Hat, Oracle or Centos
+
+ These OSs, widely distributed among commercial users, require some
+ special considerations. There is an extra document, "NGSPICE on Red
+ Hat Like Distributions.pdf", provided by Justin Fisher, available with
+ the ngspice distribution.
+
+
 2 Compilers and Options
   =====================