Add component for feature engagement tracking.

components/feature_engagement_tracker/README.md

Index: components/feature_engagement_tracker/README.md
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+# Feature Engagement Tracker
+
+The Feature Engagement Tracker provides a client-side backend for displaying
+feature enlightenment or in-product help (IPH) with a clean and easy to use API
+to be consumed by the UI frontend. The backend behaves as a black box and takes
+input about user behavior. Whenever the frontend gives a trigger signal that
+in-product help could be displayed, the backend will provide an answer to
+whether it is appropriate to show it or not.
+
+The frontend only needs to deal with user interactions and how to display the
+feature enlightenment or in-product help itself.
+
+The backend is feature agnostic and have no special logic for any specific
+features, but instead provides a generic API.