Update CONTRIBUTING.md

add a quick start section addressing common things that come up with new contributors

CONTRIBUTING.md

@@ -5,6 +5,16 @@
 We love contributions! If you're looking for a way to get involved, contact
 Sage Ross (`ragesoss`). Our main venue for collaboration is Slack; ask Sage (sage at wikiedu.org) for an invite.
 
+## Quick start guide
+
+* Set up a dev environment. Follow the [setup docs](docs/setup.md). Try the automatic scripts, and report any problems that force you to use the manual instructions. For Windows, use WSL 2 (not RubyInstaller or RailsInstaller).
+* Explore the issues, choose one to work on, and leave a comment. Work on only one issue at a time. We do not assign issues, but if it looks like someone else is working on one you are interested, give them a ping to ask if they are still active on it.
+* Open a Pull Request. Mark it as a draft if you know it is not ready to merge. Small change sets are preferred; the fewer lines changed per PR, the easier it is to think about, provide feedback on, and review. Make sure only changes related to the main issure are included; unrelated changes can be made in a separate PR. Include before/after screenshots or videos, if applicable. If you have many commits, consider squashing them into one or a few commits with well-written commit messages.
+* If you are stuck, ask on Slack.
+* If you are here because you want to apply for Google Summer of Code or Outreachy, read the [this too](docs/students_and_interns.md).
+
+## The repo and the code
+
 #### Bugs and enhancement issues
 
 The dashboard uses GitHub issues for bug tracking. Our main categories of bugs