[Docs] Add a SECURITY.md file documenting the security process.
### Motivation
In order to allow developers to report suspected security issues confidentially, we need a documented process for reporting issues affecting security.
### Modifications
Added a SECURITY.md inspired by the SwiftNIO one.
### Result
Developers can now follow the process when a suspected security issue is found.
### Test Plan
N/A
Reviewed by: simonjbeaumont
https://github.com/swift-server/swift-openapi-async-http-client/pull/1