Proposal: add a note that this library is GraalVM-friednly

[igrishaev]

First of all, thank you for that amazing library! It saved me a couple of days ago when I needed to generate an Excel file in a project that gets compiled with GraalVM. I've spent a day trying to compile POI with no result: tons of weird messages about sun.aws.XXX classes being initialized at build time... that was pure madness.

But both fastexcel and fastexcel-reader worked like a charm with GraalVM with no a single alert. I believe it's an important benefit of this project that one can using it with Graal. Thus, why not adding this note into the readme file?

Again, thank you!