Software APIs help different tools communicate with one another, let developers access essential services without having to code it themselves and are critical components for driving a platform-driven strategy. Yet they require solid documentation to help make the best use of them. ReadMe, a startup that helps companies customize their API documentation, announced a $9 million Series A today led by Accel with help from Y Combinator. The company was part of the Y Combinator Winter 2015 cohort.