J2cl: Clarify supported Java versions

[jreznot]

Hi,

Thank you for this project!

For now, I cannot find the information on supported Java versions: 8 / 11 ? It is still source to source transpiler ? Do you expect any delays with support of the upcoming Java versions ?

[gkdn]

It is stil a source to source transpiler and you can actually see the unoptimized output samples in in the transpiler's '''readable''' directory:
https://github.com/google/j2cl/tree/master/transpiler/javatests/com/google/j2cl/transpiler/readable

We tested up to Java 10 but Java 11 doesn't bring much new language features so it should be straight forward.

In general, since J2CL doesn't depend on internals of Java compiler, it is usually matter of upgrading our dependencies and testing unless there is completely new language construct like Java 8 lambdas. For Java 9, Java 10 it took couple of hours which should be true for Java 11 as well.

[jDramaix]

Let's update the README with that information