Android readme (#202)

* readme for Android SDK/NDK updates

Author: dnfield

tools/android/README.md

@@ -0,0 +1,39 @@
+# How to update Android SDK for Linux/Mac OS X on GCS
+
+## Android SDK
+
+1. Run Android SDK Manager and update packages
+  $ third_party/android_tools/sdk/tools/android update sdk
+2. Choose/Update packages
+   The following packages are currently installed:
+  - Android SDK Tools 25.2.5
+  - Android SDK platform-tools 28.0.1
+  - Android SDK Build-tools 28.0.3
+  - Android 9.0 (API 28)
+    - SDK Platform 28 rev. 6
+  - Extras
+    - Android Support Library 23.1
+    - Google Play services 49
+3. Run upload_android_tools.py -t sdk
+```
+  $ tools/android/upload_android_tools.py -t sdk
+```
+
+NOTE: If the gcloud upload fails there, it is possible to simply run
+the command manually.
+
+----------------------------------------------------------------------
+
+## Android NDK
+1. Download a new NDK binary (e.g. android-ndk-r10e-linux-x86_64.bin)
+2. cd third_party/android_tools
+  $ cd third_party/android_tools
+3. Remove the old ndk directory
+  $ rm -rf ndk
+4. Run the new NDK binary file
+  $ ./android-ndk-r10e-linux-x86_64.bin
+5. Rename the extracted directory to ndk
+  $ mv android-ndk-r10e ndk
+6. Run upload_android_tools.py -t ndk
+  $ cd ../..
+  $ tools/android/upload_android_tools.py -t ndk