Add a note that "cordova plugin" should be used for migrated projects

One user had some issues when they tried to use plugman directly
on a Cordova project they had migrated to Crosswalk.

Add a note that the recommended method for adding plugins to a
migrated project is via "cordova plugin add".

Relates to https://crosswalk-project.org/jira/browse/XWALK-1842
Add tutorial for embedding API
Link to new tutorial from embedding API tutorial page