Add feed endpoints documentation to README
Add feed endpoints documentation to README

Change-Id: Iebd69e1c2d15fa6394a0ddae115f658a5ee1bf72

wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps