Add documentation for using Swaggen 3.0.2

[mackoj]

I hope it helps.

[alexruperez]

Hi, i think /usr/local/bin has Catalina issues, doesn't it?

[mackoj]

Good point I don’t know
I don’t have a Catalina stack to test it.
It’s an optional step.
What would you advice ?

[yonaskolb]

Hi @mackoj. It's good to have a section in the readme about Swagger 2.
I'd appreciate a PR that simply makes the swagger_2 branch compile in Swift 5 though. Then we can do a 3.1.0 release