Added a readme explaining the role of each API

docs/sources/api/README.md

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+This directory holds the authoritative specifications of APIs defined and implemented by Docker. Currently this includes:
+
+* The remote API by which a docker node can be queried over HTTP
+* The registry API by which a docker node can download and upload container images for storage and sharing
+* The index search API by which a docker node can search the public index for images to download