Updated template for upcoming Scala IDE v3.0 release

* Updated project's README and explained how to create a Scala IDE plug-in in
3 simple steps.
* Updated POM used by the generated plug-in
   * Use Tycho 0.16
   * Parametrized indigo and juno builds
   * Updated build profiles for next Scala IDE v3.0 release
   * Added profile for setting strict bundle versions (needed before building
plug-in for inclusion in the ecosystem)
* Update REAMDE of the generated plug-in to reflect changes in the POM's
profile

Closes #2. Review by @dotta.