Added empty api v2 + shared folder and README.md (#9920)

refs #9866

core/server/api/README.md

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+# API Versioning
+
+Ghost supports multiple API versions.
+Each version lives in a separate folder e.g. api/v0.1, api/v2.
+Next to the API folders there is a shared folder, which the API versions use.
+
+**NOTE: v0.1 is deprecated and we won't touch this folder at all. The v0.1 folder 
+contains the API layer which we have used since Ghost was born.**
+
+## Stages
+
+Each request goes through the following stages:
+
+- validation
+- input serialisation
+- permissions
+- query
+- output serialisation
+
+The framework we are building pipes a request through these stages depending on the API controller implementation.
+
+
+## API Controller
+
+A controller is no longer just a function, it's a set of configurations.
+
+
+More is coming soon...

core/server/api/shared/index.js

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+module.exports = {};

core/server/api/v2/index.js

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+module.exports = {};