added the introduction option to the readme, and fixed the spacing on the help message for introduc…

pkg/docgen/bin/docgen.dart

Index: pkg/docgen/bin/docgen.dart
diff --git a/pkg/docgen/bin/docgen.dart b/pkg/docgen/bin/docgen.dart
index 2d75a446b61938d28a1246f96b8584472438a7e4..ca4f78054cf20d85a663ab873c019a3623be17bb 100644
--- a/pkg/docgen/bin/docgen.dart
+++ b/pkg/docgen/bin/docgen.dart
@@ -65,7 +65,7 @@ ArgParser _initArgParser() {
       defaultsTo: false, negatable: false);
   parser.addOption('introduction', 
       help: 'Adds the provided markdown text file as the introduction' 
-        'for the outputted documentation.', defaultsTo: '');
+        ' for the outputted documentation.', defaultsTo: '');
   
   return parser;
 }