Add SmallStrings

[jshchnz]

Thanks for your contribution! This is a template that can make it easier when submitting your contribution:

Your Project: SmallStrings
Project URL: https://github.com/EmergeTools/SmallStrings
Category: Optimization
Description: Reduce localized .strings file sizes by 80%.

Inclusion requirements:

• [X] Has 50 GitHub stargazers or more
• [X] Only one project/change is in this pull request
• [X] Isn't an archived project
• [X] Has more than one contributor
• [X] Has unit tests, integration tests or UI tests
• [X] Addition in alphabetical order (bottom of category)
• [X] Supports iOS 12 / tvOS 10 or later
• [X] Supports Swift 4 or later
• [X] Has a commit from less than 2 years ago
• [X] Has a clear README in English

:heart: Awesome iOS Contributors

[vsouza]

@jshchnz Thanks for contributing!