Add instructions for using the Sky SDK

These instructions don't work yet, but they're something to aim for.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/968213005

SDK.md

+Sky SDK
+=======
+
+This document describes an experimental development kit for Sky. We're still
+iterating on Sky heavily, which means the framework and underlying engine are
+both likely to change in incompatible ways several times, but if you're
+interested in trying out the system, this document can help you get started.
+
+Set up your device
+------------------
+
+Currently Sky requires an Android device running the Lollipop (or newer) version
+of the Android operating system.
+
+1. Install ``Sky`` on your device by via the Play Store. The Sky app on your
+   device is capable of displaying applications written using Sky.
+
+2. Enable developer mode on your device by visiting ``Settings > About phone``
+   and tapping the ``Build number`` field five times.
+
+3. Enable ``USB debugging`` in ``Settings > Developer options``.
+
+Set up your computer
+--------------------
+
+1. Install the ``adb`` tool from the Android SDK.
+  - Mac: ``brew install android-platform-tools``
+  - Linux: ``sudo apt-get install android-tools-adb``
+
+2. Download the Sky framework:
+  a. ``curl -O https://domokit.github.io/sky-sdk.tgz``
+  b. ``tar -xvzf sky-sdk.tgz``
+
+3. Using a USB cable, plug your phone into your computer. If prompted on your
+   device, authorize your computer to access your device.
+
+Running a Sky application
+-------------------------
+
+1. ``/path/to/sky-sdk/run myapp.sky && adb logcat``