Link to github contributing guide

[maneesha]

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

[maneesha]

This has been added to the handbook so we can link there from here:
https://docs.carpentries.org/topic_folders/maintainers/contributing.html

[maneesha]

It should also be in the CONTRIBUTING.md of all the lesson repos.

[brownsarahm]

Is the plan to incorporate the content from the guide into the handbook directly eventually or is it ok to link directly to the guide? If linking to the handbook is preferred, it also needs to be updated again in instructor-training.