Refactor scripts, so we have a single entrypoint, dartdoc.py, to generate docs.

pkg/docgen/README.md

 docgen
 ======
 
 A documentation generator for Dart.
 - - -
 The docgen tool takes in a file or directory as input and produces documentation
 for all `.dart` file it finds as YAML or JSON files. This outputs information 
 about all classes, variables, functions, and methods defined in the library and 
 its imported libraries.
 
+### Quick Start: Common Commands 
+
+While standing in the `bin` directory:
+
+`dartdoc.py` generates all documentation and runs a local server with your html
+pages.
+
+`dartdoc.py -d` ONLY generates documentation for the SDK and all packages (no
+html pages generated and no server).
+
+`dartdoc.py -d -o package/to/document` ONLY generates documenation for the
+specified package.
+
 ### Generating files & uploading to Cloud Storage
 
 The viewer uses YAML files generated by the docgen package as the data 
 being displayed. These files are stored in Google Cloud Storage.
 
  - Run `python upload_docgen.py` to generate these files and upload them to 
  Cloud Storage as a new version.
 - - -
 These tasks can be done separately if necessary:
 
@@ skipping 89 matching lines @@
  (ex. `--options='--include-sdk files'`).
  2. The `--gae-sdk` option gives the absolute path to the 
  [Google App Engine SDK][GAE]. 
 
 Running `python dartdoc.py --options=<docgen options> --gae-sdk=<path to SDK>` 
 will serve files generated by `docgen.dart` in your browser.
 
 [dartdoc-viewer]: https://github.com/dart-lang/dartdoc-viewer "Dartdoc-Viewer"
 [GAE]: https://developers.google.com/appengine/downloads#Google_App_Engine_SDK_for_Python  "Google App Engine SDK for Python"