Describe build dependencies in README

To avoid -1s on PPMC release votes, our README should include not just the build instructions but also the build dependencies, which hopefully doesn't need to use the out-of-tree code: https://github.com/awleblang/impala-setup

See:

https://lists.apache.org/thread.html/b2071858dc547ec5ab765916e5b1e722093cef4368583c0f55c586f2@%3Cgeneral.incubator.apache.org%3E

https://lists.apache.org/thread.html/d67d6059e85fd84b3297cf00565ab9b73c2aa8af06fcdfe686115ce1@%3Cgeneral.incubator.apache.org%3E