SPARK-3069 [DOCS] Build instructions in README are outdated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 The Github `README.md` contains build info that's outdated.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.)
Yeah maybe we should change the title of the doc to be called "Building Spark" and also it might be nice at the bottom to have a quick note that SBT is supported for developer builds but Maven is the reference build for all packaging. That might be the best way to do this overall.