minSdk recommendation (futurice#169)

minSdkVersion recommendation

Author: joan-domingo

README.md

@@ -47,6 +47,8 @@ Although Gradle offers a large degree of flexibility in your project structure,
 
 **General structure.** Follow [Google's guide on Gradle for Android](https://developer.android.com/studio/build/index.html).
 
+**minSdkVersion: 21** We recommend to have a look at the [Android version usage chart](https://developer.android.com/about/dashboards/index.html#Platform) before defining the minimum API required. Remember that the statistics given are global statistics and may differ when targeting a specific regional/demographic market. It is worth mentioning that some material design features are only available on Android 5.0 (API level 21) and above. And also, from API 21, the multidex support library is not needed anymore.
+
 **Small tasks.** Instead of (shell, Python, Perl, etc) scripts, you can make tasks in Gradle. Just follow [Gradle's documentation](http://www.gradle.org/docs/current/userguide/userguide_single.html#N10CBF) for more details. Google also provides some helpful [Gradle recipes](https://developer.android.com/studio/build/gradle-tips.html), specific to Android.
 
 **Passwords.** In your app's `build.gradle` you will need to define the `signingConfigs` for the release build. Here is what you should avoid: