Update contents.json

[GIKICoder]

add Markdown library

• Project Name: GMarkdown
• Project URL: https://github.com/GIKICoder/GMarkdown
• Project Description: GMarkdown is a powerful, native Markdown rendering library built on swift-markdown parser that provides seamless integration and high performance for iOS applications.
• 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 🤓