Add supported country list

[WasimMemon]

Please add list of all supported countries in description or some where.

[shayonj]

Sorry for reaching out late here. I believe providing a list of countries is directly dependent on the main API . There seems to be an open issue about that EmergencyNumberAPI/issues#3 cc @johnnye

In the meanwhile, @WasimMemon if this helps in any way - https://github.com/EmergencyNumberAPI/data/blob/master/data.json