SPARK-3069 [DOCS] Build instructions in README are outdated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 The Github `README.md` contains build info that's outdated.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.)

Author: Sean Owen <sowen@cloudera.com>

Closes #2014 from srowen/SPARK-3069 and squashes the following commits:

501507e [Sean Owen] Note that Zinc is for Maven builds too
db2bd97 [Sean Owen] sbt -> sbt/sbt and add note about zinc
be82027 [Sean Owen] Fix additional occurrences of building-with-maven -> building-spark
91c921f [Sean Owen] Move building-with-maven to building-spark and create a redirect. Update doc links to building-spark.html Add jekyll-redirect-from plugin and make associated config changes (including fixing pygments deprecation). Add example of SBT to README.md
999544e [Sean Owen] Change "Building Spark with Maven" title to "Building Spark"; reinstate tl;dr info about dev/run-tests in README.md; add brief note about building with SBT
c18d140 [Sean Owen] Optionally, remove the copy of contributing text from main README.md
8e83934 [Sean Owen] Add CONTRIBUTING.md to trigger notice on new pull request page
b1c04a1 [Sean Owen] Refer to current online documentation for building, and remove slightly outdated copy in README.md