Add miminal internal documentation

DESIGN.md

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+# Design considerations
+
+- this app tries to conform to the [Twelve Factor-App](https://12factor.net/) manifesto
+  - [currently with a known limitation according to principle VI](https://github.com/uyuni-project/hub-xmlrpc-api/issues/56)
+- this app's code is structured according to the [Clean Code Architecture](https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html)
+- this app fundamentally implements a [Gorilla](https://www.gorillatoolkit.org/) XMLRPC Server to serve its API
+- it uses [kolo/xmlrpc](https://github.com/kolo/xmlrpc) as the XMLRPC client library to consume other XMLRPC APIs
+- major packages contain a README.md file with a high-level explanation of its contents

controller/README.md

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+Controllers are called by the Codec Gorilla component and marshal/unmarshal requests and responses to functions in the gaeway package. There is one Controller per supported method or namespace.
+
+Functions in the gateway package actually implement that method or namespace's functionality.

controller/parser/README.md

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+Parsers validate parameters to individual XMLRPC methods/namespaces supported by this app, each being implemented by the Controller with the same name.

gateway/README.md

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+Functions in this package implement supported XMLRPC methods/namespaces, and they are normally called by Controllers.

session/README.md

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+Implementation of backing storage for session data.

uyuni/README.md

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+Implementation of functions specific to Uyuni.