docs: add env var documentation

README.md

@@ -124,3 +124,15 @@ Example:
 ```sh
 /audit-pre-release 8-x-y
 ```
+
+## Environment Variables
+
+If you would like to use `unreleased`, there are several environment variables you will need to leverage to customize it to serve your needs.
+
+* `ORGANIZATION_NAME` - the name of your organization, e.g. `electron`.
+* `REPO_NAME` - the name of the repository to run audits in, e.g. `electron`.
+* `NUM_SUPPORTED_VERSIONS` - the number of supported backport release lines (default is 4).
+* `GITHUB_TOKEN` - the token that will be used to fetch data from the GitHub API (required).
+* `SLACK_BOT_TOKEN` - the token that will be used to post audit result into a Slack workspace channel (required).
+* `BLOCKS_RELEASE_LABEL` - the GitHub label used to denote unmerged pull requests that should block a release (default is `blocks-release`).
+* `AUDIT_POST_CHANNEL` - the Slack workspace channel in which to post audit results.