Improve README with AWS API Gateway findings

[fahernandez]

After trying to import swagger generated documentation into AWS API gateway I discover some helpful information that can be shared in README file to help other people through this process:

1. Remove model circular dependencies.
2. Remove security-related annotations (This annotation aren't well supported by the import functionality).
3. Max length of the fields is reviewed by the parser but the errors aren't self-explanatory. Review the documentation to verify that the requirements are met.
4. API gateway errors aren't good enough for troubleshooting so I recommend using this page for structure validation.

[fahernandez]

Documentation was added on this pull request

[johanbrandhorst]

Closed by #869