Add more info in regards impossibility to compile the lib with multicore feature to wasm targets

[CPerezz]

Derived from #116 and as discussed offline with @huitseeker, We should add some README section that explains that the lib (as any Rust lib) that uses rayon as a dependency cannot compile to a wasm-target.

Not only that, but I think it will also be beneficial to add a compiler warning such that the user gets some hint on what the source of the error is.

[CPerezz]

Resolved by #122