Add code of conduct (#2272)

Due to the migration, the code of conduct is missing since we don't have access to the global rapids org file https://github.com/rapidsai/.github/blob/main/CODE_OF_CONDUCT.md

Instead we can re-add it as a file in the .github folder of this repo.

Authors:
  - Anupam (https://github.com/aamijar)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #2272

Author: aamijar

.github/CODE_OF_CONDUCT.md

@@ -0,0 +1 @@
+This project has adopted the [Contributor Covenant Code of Conduct](https://docs.rapids.ai/resources/conduct/).