README and NEWS update

[ci skip]

Author: tute

Diff for: NEWS

@@ -1,5 +1,7 @@
 master:
 
+5.1.0 (2016-08-19):
+
 * Add default `content_type_detector` to `UploadedFileAdapter` (#2270)
 * Default S3 protocol to empty string (#2038)
 * Don't write original file if it wasn't reprocessed (#1993)

Diff for: README.md

@@ -30,6 +30,7 @@ https://github.com/thoughtbot/paperclip/releases
   - [Edit and New Views with Simple Form](#edit-and-new-views-with-simple-form)
   - [Controller](#controller)
   - [View Helpers](#view-helpers)
+  - [Checking a file exists](#checking-a-file-exists)
   - [Deleting an Attachment](#deleting-an-attachment)
 - [Usage](#usage)
 - [Validations](#validations)