* Add deprecation notice to README

Addresses new projects, existing projects, issues, and PRs.

So that the `file_system` option renders correctly on the docs

Closes https://github.com/thoughtbot/paperclip/issues/2253

Follow #2481

2254 coverage (#2395)

* f(x): default to Paperclip::ContentTypeDetector

* fix: failures

* spec: cover consecutive periods

* fix: hound

* fix: hound

- open-uri is a security concern, so let's not recommend it. Separate
one-liners based on whether they're doing local or remote storage.
- Mention the performance impact of the separate table. While migrating
they will want to consider each use case for n+1 loads.

Thanks, Derek!

A start at an introductory migration path for moving from Paperclip to
[ActiveStorage].

ActiveStorage is coming in Rails 5.2 (April, perhaps) and we want to
help people move from Paperclip to it. Having a file upload strategy
baked into Rails itself will allow us all to focus our efforts and
follow a common goal.

This migration guide is rudimentary at best: it works for a `User`
object with one `avatar` attachment. It is provided as a framework to
start the documentation.

I have tried this in a dummy app.

[ActiveStorage]: http://edgeguides.rubyonrails.org/active_storage_overview.html

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* Added support for aws-sdk-s3 gem which is now preferred way to interact with s3.

Reference: https://github.com/aws/aws-sdk-ruby/blob/master/V3_UPGRADING_GUIDE.md#library-maintainer

* Drop support for aws-sdk gem

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There was a bug in the code where if you created a bucket the file would
get read but not rewind on the first pass through (when the bucket
didn't exist). So when we went into the retry logic the file would
already be read and zero bytes would get copied over from the file. This
fixes that by rewinding the file during the retry so it gets correctly
copied over the second time around.

Please see #2527 for details.

The command from the README is often copied into projects directly. The version used here before had a known vulnerability (CVE-2017-0889), so it might be wise to mention something more secure instead.

* Provide example for s3_host_name

Gives an example, since not everyone knows the format required.

* Shorten line per Hound comment

* Added convert_options and source_file_options documentation to the Post Processing README section

* made minor markdown changes

* Add migration_version variable to migration generator for rails 5 compatibility

* Update syntax to satisfy hound

* Ruby 2.4.0 compatibility (doen't use Fixnum anymore)

* Fix typo

* Use Integer instead of 0.class

* Updated the Readme, added a submit button to the form and made some notes to make instructions more clear

Use consistent code style in README examples

Update README.md

Fix Homebrew hyperlink on README

Rename FactoryGirl to FactoryBot

[ci skip]

This is a year in the making, with a security patch motivating it.

[ci skip]