Android example README inaccurate

[c-parsons]

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The readme for the android app tutorial target reads:

"Once these are done, set up the android_sdk_repository rule in the WORKSPACE file, following the instructions in the comments there."

There are no such instructions nor inline comments in the WORKSPACE file.

As such, I can't build //android. I'm undoubtedly missing some SDK configuration step.

[c-parsons]

For what it's worth, I was missing the enviroment variable ANDROID_HOME, which is required as is described in the android_sdk_repository documentation However, this was quite unclear given the error messages and the README.