Improve README of main project

[pepijn]

Should contain:

• Overview of the different subprojects: ring adapter, background adapter, commons.
• Recommendation to use the ring adapter in conjunction with pubsub.
• Introduction to the Pubsub middleware.
• Instructions on deployment and explain the javac compilation.
• Add a dependency diagram.
• Explaining that GCF is like Cloud Run so the function 'stays warm' between requests (until a certain timeout).

Explain why using namespaced maps

https://clojure.org/guides/spec

For example, most Ring middleware functions modify the request or response map with unqualified keys. However, each middleware could instead use namespaced keys with registered semantics for those keys. The keys could then be checked for conformance, creating a system with greater opportunities for collaboration and consistency.