Auto merge of #11360 - mmatyas:readme_androidbuild, r=Manishearth
Auto merge of #11360 - mmatyas:readme_androidbuild, r=Manishearth

Update Android build instructions in README

The Android build instructions haven't been updated for a while in the README;
instead of `cd ports/android && ANDROID_SDK=/path/to/sdk make install`, we have `./mach package --release` now.

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