Add minimal docs to the NPM library

[mxdvl]

It could help adoption by JS consumers.

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So this is how one use the models from content-api-models https://github.com/guardian/dotcom-rendering/blob/8c46c6b669b600b439c5585f99678fee75a7a451/apps-rendering/src/capi.ts#L6

Basically you just import the model you would like

The elements you can find are in the thrift file
https://github.com/guardian/content-api-models/blob/main/models/src/main/thrift/content/v1.thrift#L1897