Update README.md

Added changes to instructions to work with Rails 4's strong parameters.
Clarify Rails 3/4 things in the README
Deep merge Attachment options into default_options
failing spec for attachment definitions
traverse ancestors to find definitions
Refactoring of previous commit
Update link in documentation for :s3_permissions.
document :s3_credentials being able to take a Proc
Remove references to closed mailing list

http://robots.thoughtbot.com/moving-open-source-project-mailing-lists-to-stack-overflow/
Merge pull request #1402 from thoughtbot/jf-remove-mailing-list

Remove references to closed mailing list
Merge pull request #1400 from thehenster/master

document :s3_credentials being able to take a Proc
Update regexes to bind to beginning and end of string

This ensures that multiline strings are properly matched in their
entirety.
Update README to stress that content_types should be validated as a rule
slice for conditional lambda and add to options
Use :swallow_stderr instead of not thread safe silence_stream(STDERR)
Delegate close! to @tempfile
Test to prevent regression of previous commit
Getting rid of some deprecations and ruby warnings
Fix a strange new failure in 1.9.2/3.2
Preserve files when reprocessing

When using S3 for storage, their data consistency model makes the
current behaviour of Attachment#reprocess! unreliable.

Apparently, Amazon can not guarantee that PUT's called after DELETE's
is actually executed in that order.

Hence, I propose preserving files, when doing a reprocces.