SPARK-3069 [DOCS] Build instructions in README are outdated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 The Github `README.md` contains build info that's outdated.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.)
Similarly here, I think `sbt` is the default way to do these things, and Maven is a supported alternative. 

It's confusing that the [main site](http://spark.apache.org/docs/latest/building-with-maven.html) only lists Maven for building Spark, but since it's listed as "Building Spark with Maven" as opposed to just "Building Spark" I read that as an alternative way of building that is supported, rather than the main way.