Adds API policy doc

Signed-off-by: Shivam Mukhade <smukhade@redhat.com>

docs/API_POLICY.md

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+# Hub API Policy
+This document proposes a policy regarding making API updates in this repo.
+
+There are 2 types of APIs:
+1. Versioned (For External Users): These are the APIs which are for users to use and will have support provided by the Hub team. These are the APIs which return tekton resources data. These are versioned APIs, the version of the APIs will be prefixed to the API route. 
+Eg. resource API - /v1/resources
+2. Internal APIs: These are the APIs which are specifically used for Hub UI and hub services. These are not exposed for integration.
+Eg. Login, rating, Catalog refresh APIs, etc.
+
+### User facing APIs
+- These APIs can be used for integration for users.
+- These APIs are versioned and have prefixed their version. Eg. /v1
+
+#### Additive changes
+Additive changes are changes that add to the API and do not cause problems for users of previous versions of the API.
+These changes must be approved by at least 2 OWNERS.
+
+#### Backwards incompatible changes
+Backwards incompatible changes change the API, eg. removing a field in API response or removing an API.  These changes will mean that folks using a previous version of the API will need to adjust their usage in order to use the new version or an alternate API.
+
+#### Deprecated Features
+These are the features which will be removed in the next release but for folks using the previous  version would be able to use the feature for some amount of time.
+Eg. If a feature is deprecated from v2 to v3, then v2 APIs would be supported for x amount of time.
+
+### Internal APIs
+These APIs are used for Hub UI and internal hub service.
+These may change at any time as the Hub UI evolves.
+No support is provided for integration of this APIs. 
+
+