Updated template for upcoming Scala IDE v3.0 release

What's New
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* 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 README of the generated plug-in to reflect changes in the POM's profile

Note 
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This should be merged only **after** v3.0.0 final is announced.