Update README to include information on adding Background Functions to a site

[ericapisani]

It's not entirely clear how to set up Background Functions to a Gatsby site.

We should add a section to this project's README telling users how to do so

Related issue: #436

Related documentation:

• https://docs.netlify.com/functions/background-functions/
• https://docs.netlify.com/functions/build-with-javascript/#background-function-format

[ascorbic]

Might as well add info for scheduled functions at the same time, as the process is the same