SPARK-3069 [DOCS] Build instructions in README are outdated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 The Github `README.md` contains build info that's outdated.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.)
@srowen I think it's a good idea to have a CONTRIBUTING file here, for the reasons you explained elsewhere, like [this one](https://github.com/blog/1184-contributing-guidelines). I'd actually favor moving the [Contributing to Spark](https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark) page entirely out of the wiki and into here. 

I believe Spark accepts contributions entirely through GitHub, so it makes sense to have the contributing instructions live here.