Update README.md with instructions to edit systems.csv (#683)

## What's Changed
This PR adds instructions in the repo README.md on how to add an entry or edit systems.csv.

It's the same instructions as in [gtfs-validator](https://github.com/MobilityData/gtfs-validator/blob/master/docs/CONTRIBUTING.md#submitting-a-pull-request-contribution).

README.md

@@ -114,8 +114,16 @@ Caution should be taken to avoid making changes to the spec that would render ex
 Each new addition to the spec adds complexity. We want to avoid additions to the spec that do not provide additional value to the shared mobility end user.
 
 ## Systems Catalog - Systems Implementing GBFS
-There are now over 900 shared mobility systems publishing GBFS worldwide. This list contains all known systems publishing GBFS feeds and is maintained by the GBFS community. This is an incomplete list. If you have or are aware of a system that doesn’t appear on the list please add it.
-If you would like to add a system, please fork this repository and submit a pull request. Please keep this list alphabetized by country and system name. Alternatively, fill out [this contribution form](https://forms.gle/WSXFuXx9k4PSTfbC9) for a Github-less contribution. 
+There are hundreds of shared mobility systems publishing GBFS worldwide. This list contains all known systems publishing GBFS feeds and is maintained by the GBFS community. This is an incomplete list. If you have or are aware of a system that doesn’t appear on the list please add it.
+
+If you would like to add a system, please fork this repository and submit a Pull Request. To open a Pull Request, please do the following:
+
+- [Create an account on GitHub](https://github.com/join) if you do not already have one
+- [Fork](https://docs.github.com/en/get-started/quickstart/fork-a-repo) this repository
+- Create a new branch, and
+- Propose your changes by opening a [new pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests)
+
+Please keep this list alphabetized by country and system name. Alternatively, fill out [this contribution form](https://forms.gle/WSXFuXx9k4PSTfbC9) for a Github-less contribution. 
 * [systems.csv](systems.csv)
 
  Field Name | REQUIRED | Definition