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 114x14px 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.