Add proguard settings

[zugaldia]

We need to add details on the required proguard settings.

Next steps:

• Add settings @zugaldia
• Document @cammace

/cc: @bleege

[zugaldia]

@cammace Proguard rules are in https://github.com/mapbox/mapbox-java/blob/master/libandroid/lib/proguard-rules.pro so probably we don't need documentation, or just a link to that file if someone is curious about the rules.