Add Tutorials for Mobile

[eXpl0it3r]

iOS and Android have been supported with limits (e.g. no shader support due to missing OpenGL ES 2.0 support) and deserve a dedicated tutorial on how to use SFML for those platforms.

Old-ish tutorials on the wiki: Android / iOS

Include tips on how to:

• Handle window creation
• Handle putting apps into background (see also CPU usage on Android skyrockets once an SFML app is in background SFML#634 (comment) )
• ...