README: state that it works better with Android 4.4

I have tried with Android 4.2.2 on a MK 809 III TV stick and the webview
is far more limited. Moreover, I had several segfaults. Maybe this
should be investigated nonetheless.

A solution could be to embed Chromium WebView. See this GitHub project:
 https://github.com/pwnall/chromeview

Another solution is to switch to GeckoView:
 http://starkravingfinkle.org/blog/2013/10/geckoview-embedding-gecko-in-your-android-application/