Add a changelog

[singhprd]

Hey there 👋

Thanks for the work from the maintainers on this Gem 👏

I've been updating the version of date_validator in our application, and have spent some time debugging why some custom error messages have changed.

I tracked this down to changes to the locale keys, and made the appropriate changes in my application 🎉

Something that would have made this easier however, for me and others, would have been to check a change log for the project, so I propose this is added at the project root?

I've drafted a quick starter based on what I've read from the 0.10.0 release here:

# Changelog
All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).

## Unreleased

## 0.10.0 - 2020-03-03
### Changed
* Breaking changes to locale keys.
* Drop Ruby 1.9.x support, minimum Ruby version now >= 2.2
* Change locale path to Rails Engine default

Hope this helps!

• https://keepachangelog.com/en/1.0.0/

[be-ragheb]

Since I had to review changes anyway, I took my notes here in this comment box and I'm posting them. Someone with deeper knowledge of the project should be able to clean this up. I'd open a PR, but I work for the kind of company that won't let me do that without a lot of filling out of forms.

0.12.0

• added MIT license to the gemspec
• clarified support for Ruby 2.2 and later, including 3.0

0.11.0

• fixed Simplified Chinese
• fixes an issue with Ruby 3 and Rails 6.1, see Spread keyword args to support Ruby 3 #85

0.10.0

• added Simplified Chinese, Arabic
• updated Spanish, Catalan
• Breaking changes to locale keys.
• Drop Ruby 1.9.x support, minimum Ruby version now >= 2.2
• Change locale path to Rails Engine default

0.9.0

• added ActiveSupport dependency
• added Turkish locale
• don't treat invalid strings as valid; fixes Fail to detect errors when invalid strings are provided #48
• fixed Rails initialization

0.8.1

• Fixed typos in Japanese locale
• Fixed i18n load path

0.8.0

• Added or updated translations for Afrikaans, Italian, Japanese, Catalan
• Load locales correctly; fixes Complain of missing translation #44