Link to github contributing guide

In a conversation on slack, a community member shared this guide on how to contribute to Carpentries lessons. @ErinBecker suggested it go in the handbook and other places including lesson-example.

Guide:
https://github.com/dmgt/swc_github_flow/blob/master/for_novice_contributors.md

Review the linked contributing guide and inspect the lesson-example repository's existing handbook or contributor documentation to identify the requested location. Done means the guide is linked in the agreed lesson-example documentation; confirm with maintainers where the other requested placements belong.

Written by the indexing model from the issue text.