Hi yt-dev!

Over the past few days at the yt development workshop we added some new bots/features to the yt repository that will hopefully help streamline some aspects of the maintenance of the project. Here is what we came up with so far:

- We added a welcome bot that welcomes and thanks new contributors to the project on their first issue or PR. 
- We added a triage bot that adds a "triage" label to all new issues. Once they have been triaged and relevant labels have been added, the bot automatically removes the triage label. 
- We added a few "projects" to the projects tab of the repo (yt 4.0, weather data, and frontend refactor). The intent here is to document what needs to be done and track our progress on projects/milestones that we have for yt. 
- We added new labels for issues and PRs. We've added domains, index, and coordinates labels to help refine our classifications of different issues. I also updated the coloring on some of these labels, so 
  * greens: things that are direct improvements or are related to new contributors 
  * browns: related to project infrastructure / tests / docs
  * blues: future ideas / aspirations
  * yellows: awaiting actions
  * everything else is random

If any of you have feedback/concerns/other ideas on how we can improve what I've mentioned here, ideas on what else can be done to streamline things, or declaring your love for different color palettes, please chime in! These are intended to be a helpful starting point, not random unilaterally-decided additions to the project.