Added a flag package-root for users to define where the package directory is.

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 f181a836dec9092f2bd244eb807809a53d6f5bc4..df6ce632d55f20297103d3a7db5b599122fe06e3 100644
--- a/pkg/docgen/bin/docgen.dart
+++ b/pkg/docgen/bin/docgen.dart
@@ -55,6 +55,8 @@ ArgParser initArgParser() {
   parser.addFlag('parse-sdk', 
       help: 'Parses the SDK libraries only.', 
       defaultsTo: false, negatable: false);
+  parser.addOption('package-root', 
+      help: "Sets the package root of the library being analyzed.");
   
   return parser;
 }