Add FixFlex

[psharanda]

• Project Name: FixFlex
• Project URL: https://github.com/psharanda/FixFlex
• Project Description: Declarative autolayout based on NSLayoutAnchor, swifty reimagination of VFL, alternative to UIStackView
• Why it should be added to awesome-swift:
• At least 15 stars (GitHub project)
• Support Swift 5
• Updated contents.json instead of README
• Lib is fully open sourced, written in Swift and not a wrapper over compiled lib
• Description does not say "written in Swift" or variant 🤓

[vpeschenkov]

@psharanda thanks 👍