docs(contributing): add CONTRIBUTING with all-contributors instructions

Author: amyheather

CONTRIBUTING.md

@@ -0,0 +1,37 @@
+# Contributing
+
+## `all-contributors`
+
+If your name or contributions are missing from the README, or if you contributed in ways not captured by the current role emojis, please create an issue and use: 
+
+```
+@all-contributors please add @githubuser for ...
+```
+
+Then list appropriate contribution types from [allcontributors.org/docs/en/emoji-key](https://allcontributors.org/docs/en/emoji-key) (e.g., code, review, doc, content, bug, ideas, infra).
+
+Alternatively, you can update it from the command line. This may be preferable, as the bot will create GitHub issues that email people when they are added.
+
+You'll need to install the [All-Contributors CLI tool](https://allcontributors.org/cli/installation/):
+
+```
+npm i -D all-contributors-cli
+```
+
+When we first set-up all-contributors, we called:
+
+```
+npx all-contributors init
+```
+
+From then on, you can then run the following and select/enter relevant information when prompted:
+
+```
+npx all-contributors
+```
+
+If you want to remove specific contributions or people, edit the `.all-contributorsrc` file then run the following to regenerate the table in `README.md`. (Don't edit `README.md`, as it is just generated based on `.all-contributorsrc`).
+
+```
+npx all-contributors generate
+```