Enhance Eclipse usage instructions

Updates to instructions for setting up DbFit in Eclipse.
Yes, we could just say execute `./gradlew eclipse` from the top level dbfit directory. 

I don't know about for IDEA though - does that need to be run from `dbfit-java`.

I think there's no issue with the commands for each being run from different dirs.

I do need to update the doc to say to also ignore the `FitNesseRoot` project that's hanging around from the old SVN repo *unless we should delete this now?*