docs: add maintainers doc

docs/MAINTAINERS.md

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+# Maintainers
+
+## List of current maintainers
+
+The following people are the current maintainers. Feel free to reach out to them for any questions.
+
+- [Dominik Kundel @dkundel](https://github.com/dkundel)
+  - Pronouns: he/him
+  - Twitter: [@dkundel](https://twitter.com/dkundel)
+  - Email: [dkundel@twilio.com](mailto:dkundel@twilio.com)
+- [Tilde Ann Thurium @annthurium](https://github.com/annthurium)
+  - Pronouns: they/them
+  - Twitter: [@annthurium](https://twitter.com/annthurium)
+  - Email: [tthurium@twilio.com](mailto:tthurium@twilio.com)
+- [Lizzie Siegle @elizabethsiegle](https://github.com/elizabethsiegle)
+  - Pronouns: she/her
+  - Twitter: [@lizziepika](https://twitter.com/lizziepika)
+  - Email: [lsiegle@twilio.com](mailto:lsiegle@twilio.com)
+
+## Process Steps For Maintainers
+
+The project is powered by [GitHub Actions](https://github.com/features/actions) and ideally Pixel Contributions should be merged automatically by [`mergify`](https://mergify.io/) but sometimes things go wrong and we need to manually review.
+
+![screenshot of GitHub checks output with arrows pointing at two details links for different tasks](images/github-actions-check.png)
+
+These are some guides on how to deal with those PRs
+
+### Pixel Contributions
+
+1. Make sure the person is a first-time contributor. There is a badge at the top of the PR
+   ![screenshot of the GitHub PR interface highlighting the `First-time contirbutor` badge](images/first-time-screenshot.png) 1. If they are not a first-time contributor but are contributing a pixel, make sure they do not have another merged PR with a pixel. You can either search the PRs or look in `_data/pixels.json` file.
+2. Make sure the pixel contribution is valid in the first place by checking the `Files changed` tab
+   - there should not be any other files touched
+   - the coordinates of the pixel should be less than `40`
+     - if this is the case, you can use the following message template to request changes
+       > Hey @dkundel it seems like that someone unfortunately already grabbed the pixel you are trying to contribute. Could you please [sync your fork](https://help.github.com/en/articles/syncing-a-fork) and contribute a different pixel instead? Thank you for your contribution!
+   - the `username` should be the same as their GitHub username they are using to open the PR
+   - they are not overriding another person's pixel
+     - if this is the case, you can use the following message template to request changes
+       > Hey @dkundel it seems like that you grabbed the pixel that is outside of the canvas. Any coordinate needs to be smaller than 40. Could you please [sync your fork](https://help.github.com/en/articles/syncing-a-fork) and contribute a different pixel instead? Thank you for your contribution!
+3. Check if the PR has a merge conflict. You can see it at the bottom of the page. If there is a merge conflict:
+   1. Press the `Resolve conflict` button
+   2. Generally press `Accept Current`. The only exception is if the conflict involves the user who created the PR. In that case:
+      - Copy their pixel contribution
+      - `Accept Current` instead
+      - Paste the pixel contribution into the right place
+   3. Once all merge conflicts have been resolved, scroll to the top and finish the resolving at the top right by committing the changes
+   4. Wait for the checks to perform again. If everything succeeds the bot should automerge
+4. If everything looks fine until now, open the `Details` part of the `Code Review / build (pull_request)` and make sure that everything passes except `Danger`. If `Danger` failed with a message that some pixels are being deleted but you can't find any removed lines in the `Files changed` tab, that means their PR is just out of sync. Feel free to squash the PR manually if everything else passes.
+
+### Non-Pixel Contributions
+
+1. Add yourself as a reviewer to claim the PR
+2. Make sure the changes build and deploy. You can see this if the `deploy/netlify` task is marked green.
+   a. [if applicable] Click the `Details` link for the `deploy/netlify` link to see the deploy preview page. This allows you to manually test the changes on the website
+3. Make sure Unit Tests pass by clicking the `Details` link on `Code Review / build (pull_request)`. The `Danger` part of this is intended to fail, but every other task especially `npm install, build and test` should pass
+4. Review the code and request any changes if necessary. Otherwise click the `Approve` button under `Review changes` in the `Files changed` tab
+5. Squash the changes and if necessary update the commit message to fit the [conventional commit message style](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit)
+6. Make sure any related issues are closed