Remove ReadmeService

Migrating to WelcomeService in SciJavaCommon
Add WELCOME.md

Adds the file that will be displayed when IJ2 is first run.
Merge pull request #84 from imagej/remove-readme-service

Removes the ReadmeService, which has been replaced by a WelcomeService at the SciJava-Common level. We now provide a WELCOME.md which allows a separation of information - README.md for developers looking at information, e.g. on GitHub, and the WELCOME.md for introducing users to ImageJ2.