Clarify goal of project and alternatives in README

[chuwy]

I think it could be useful to provide new users a little note that they currently can use https://github.com/cquiroz/scala-java-time as drop-in replacement for java.time, whereas this project intended to be only JS implementation.

I got this information from this lengthy thread, but maybe I misenterpret something.

[rpiaggio]

If I may place a suggestion, it could also be mentioned that for a lightweight alternative, https://github.com/zoepepper/scalajs-jsjoda is also availbable.