Set minSdkVersion to 21 (Android 5.0) (#976)

* Set minSdkVersion to 21 (Android 5.0)

* Update README

README.md

@@ -2,6 +2,9 @@
 
 **Supported Server Versions:** 4.0+
 
+**Supported iOS versions:** 9.3+
+**Supported Android versions:** 5.0+
+
 Mattermost is an open source Slack-alternative used by thousands of companies around the world in 11 languages. Learn more at https://mattermost.com.
 
 You can download our apps from the [App Store](https://about.mattermost.com/mattermost-ios-app/) or [Google Play Store](https://about.mattermost.com/mattermost-android-app/), or package them yourself. 

android/app/build.gradle

@@ -93,7 +93,7 @@ android {
 
     defaultConfig {
         applicationId "com.mattermost.rnbeta"
-        minSdkVersion 16
+        minSdkVersion 21
         targetSdkVersion 23
         versionCode 52
         versionName "1.3.0"