[DOCS][BUILD] Add instruction to use change-version-to-2.11.sh in 'Building for Scala 2.11'.

To build with Scala 2.11, we have to execute `change-version-to-2.11.sh` before Maven execute, otherwise inter-module dependencies are broken.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3361 from ueshin/docs/building-spark_2.11 and squashes the following commits:

1d29126 [Takuya UESHIN] Add instruction to use change-version-to-2.11.sh in 'Building for Scala 2.11'.

(cherry picked from commit 0fcd24cc542040ff3555290eec7b021062e7e6ac)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>