Update contributing instructions to add acknowledgements

Author: rgommers

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 Thank you for your interest in contributing!
 
 
-## Contributing 
+## Contributing
 
 If you would like to contribute textual clarifications or other "maintenance
 type" changes, please open a pull request on the
@@ -17,10 +17,21 @@ before opening a pull request. If your proposed addition seems in scope,
 opening an issue for discussion may be more appropriate as a first step.
 
 
+## Acknowledgements
+
+We recognize all types of contributions. This project follows the
+[all-contributors](https://github.com/all-contributors/all-contributors)
+specification. Please add yourself to the list of contributors when you make
+a contribution. You can do this by posting a comment with the text:
+`@all-contributors please add @YOUR-USERNAME for THING(S)` (`THING` is one of
+[these keys](https://allcontributors.org/docs/en/emoji-key)) and our nice bot
+will add you.
+
+
 ## Reporting issues
 
 Please use the [issue tracker](https://github.com/data-apis/array-api/issues)
-for reporting issues with the published Array API Standard. 
+for reporting issues with the published Array API Standard.
 
 Please note that *implementation* issues should be reported to the project
 implementing the standard.