Update README.md with "Build From Source" section

README.md

@@ -19,3 +19,42 @@ Major contributions to Lens Launcher have been made by Rish Bhardwaj (@CreaRo)
 
 You can download Lens Launcher on Google Play:
 https://play.google.com/store/apps/details?id=nickrout.lenslauncher
+
+## Build from Source
+
+If you'd like to build from source, you may do so easily with [Docker][1], but
+if you are wanting to contribute it may be better to install the build tools on
+your machine.
+
+### Easy Way
+
+Building the APK is extremely easy using [`docker-compose`][2], all that is
+needed is to run the following inside the repo's directory:
+
+```bash
+docker-compose run --rm gradle
+```
+
+Then download the APK inside
+`lens-launcher/app/build/outputs/apk/app-release.apk` to your phone to install.
+
+### For Development
+
+1.  Install the latest Android SDK from [Android Studio][3] and accept the
+    licenses.
+2.  Install Java JDK 8 from [Oracle][4].
+3.  Run `keytool` and create a keystore following the command prompts.
+4.  Generate an APK with `gradlew` like below:
+
+    ```bash
+    gradlew assembleRelease \
+      -Pandroid.injected.signing.key.alias=$KEY_ALIAS \
+      -Pandroid.injected.signing.store.file=$KEY_STORE_PATH \
+      -Pandroid.injected.signing.store.password=$KEY_STORE_PASSWORD \
+      -Pandroid.injected.signing.key.password=$KEY_PASSWORD;
+    ```
+
+[1]: https://docs.docker.com/
+[2]: https://docs.docker.com/compose/install/
+[3]: https://developer.android.com/studio/index.html
+[4]: http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html