Add requirements section on README.md

[mugbug]

Add those minimum required versions to use this package to README.md

• OS versions
• Xcode version
• Swift version

[hamzzy]

I would love to work on this can you give a brief of what the requirement should contain

[mugbug]

@hamzzy done!

[hamzzy]

Okay on it

[hamzzy]

done #29