Document what privileges the Coveo API key needs to have.

Problem/Motivation

Document what privileges(permissions) are needed on the Coveo API key for this module to work.

This could be added into the project's readme.

It looks like more than the "Push items to sources" privledge is needed because this module also makes calls to https://platform.cloud.coveo.com/rest/organizations/{organizationId}/indexes/fields/search

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes