Add instructions for installing doxygen on RHEL/Fedora/CentOS

tasks/doc.rake

@@ -18,7 +18,10 @@ namespace :doc do
       sh "doxygen Doxyfile"
     rescue
       puts "ERROR: To generate C API documents, you need Doxygen."
+      puts "On Debian-based systems:"
       puts "  $ sudo apt-get install doxygen"
+      puts "On RHEL-based systems:"
+      puts "  $ sudo dnf install doxygen"
     end
   end