Improvement readme.md

[spottuit]

So I have a suggestion:

• add the configuration line below because it isn't added to the config.xml automaticly. I had to go to https://build.phonegap.com/plugins/2421 to get the line, but before that I had to search alot to find it.

 <gap:plugin name="com.phonegap.plugins.barcodescanner" version="2.2.0" /> 

EDIT: I removed the links because they are working here. They didn't work in a forked repo.