SPARK-3069 [DOCS] Build instructions in README are outdated

Here's my crack at Bertrand's suggestion. The Github `README.md` contains build info that's outdated. It should just point to the current online docs, and reflect that Maven is the primary build now.

(Incidentally, the stanza at the end about contributions of original work should go in https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark too. It won't hurt to be crystal clear about the agreement to license, given that ICLAs are not required of anyone here.)
Per [the related discussion here](https://github.com/apache/spark/commit/73b3089b8d2901dab11bb1ef6f46c29625b677fe#commitcomment-7552507), I don't think we want to change this. There are definitely two ways on offer to build Spark, but I think `sbt` is the default way to go and Maven is supported alongside it.

Perhaps we should just clarify this here.