Update README.md

* Add link to the contributor list from GitHub 
* Remove reference to macthemes forum, as the original topic is apparently not indexed by google aymore
* Add advice on where to find missing metadata
* Add Google images tip to find icon graphics
* In guidelines only keep PNG as accepted format (am I right?)
* Icon size should be 114x114px at a minimum (not 512)
* Orientation : icon must be centered within the guides template

**Note** : I don't know if the statement "_Note: There is no maximum size (although 1024px by 1024px is plenty)._" is still valid.