servo: Merge #11360 - Update Android build instructions in README (from mmatyas:readme_androidbuild); r=Manishearth

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.

Source-Repo: https://github.com/servo/servo
Source-Revision: e18bf819059e4439d276b02083a60f6353aaa359

servo/README.md

@@ -143,9 +143,14 @@ real-world use, add the `--release` flag to create an optimized build:
 ``` sh
 git clone https://github.com/servo/servo
 cd servo
-ANDROID_TOOLCHAIN=/path/to/toolchain ANDROID_NDK=/path/to/ndk PATH=$PATH:/path/to/toolchain/bin ./mach build --android
-cd ports/android
-ANDROID_SDK=/path/to/sdk make install
+
+export ANDROID_SDK="/path/to/sdk"
+export ANDROID_NDK="/path/to/ndk"
+export ANDROID_TOOLCHAIN="/path/to/toolchain"
+export PATH="$PATH:/path/to/toolchain/bin"
+
+./mach build --release --android
+./mach package --release
 ```
 
 Rather than setting the `ANDROID_*` environment variables every time, you can