README: Add entry for the Agenda View.

* README (Agenda View): New entry.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Agenda View: Provide hints on error page.

* src/org-ehtml-server.el (org-ehtml-serve-file): Provide hints if
  agenda command is unknown.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #6 from ruediger/agenda2

README: Agenda