[nearby] Update README to add build env script

Test: n/a
Ignore-AOSP-First: nearby_not_in_aosp_yet
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:85afaab1ac308f12d7263ef4b8a2b1ba6a3a2f13)
Merged-In: I43257c79d212ed24fb0cd69f363b25bceb098f01
Change-Id: I43257c79d212ed24fb0cd69f363b25bceb098f01
1 file changed