S3-compatible object storage API (#2720)

* s3 active_storage

* updated readme

config/storage.yml

@@ -13,6 +13,13 @@ amazon:
   secret_access_key: <%= ENV['AWS_SECRET_ACCESS_KEY'] %>
   region: <%= ENV['AWS_REGION'] %>
   bucket: <%= ENV['AWS_BUCKET'] %>
+s3:
+  service: S3
+  endpoint: <%= ENV['S3_ENDPOINT'] %>
+  access_key_id: <%= ENV['S3_ACCESS_KEY_ID'] %>
+  secret_access_key: <%= ENV['S3_SECRET_ACCESS_KEY'] %>
+  region: <%= ENV['S3_REGION'] %>
+  bucket: <%= ENV['S3_BUCKET'] %>
 
 # Remember not to checkin your GCS keyfile to a repository
 google: