Add setup info to Eclipse plugin page

I rearranged your words, so please take a look at the diffs:
https://chromiumcodereview.appspot.com/14935010/diff2/1:2001/src/site/tools/eclipse-plugin/index.html

This change is staged at:

http://eclipse.dart-lang.appspot.com/tools/eclipse-plugin/

Review URL: https://chromiumcodereview.appspot.com/14935010