Add link to GitHub Actions documentation for GITHUB_TOKEN

Not understanding the GITHUB_TOKEN cost me a fair bit of time recently.
Hopefully adding this link to the docs will save time to other people.

See
danger#856 (comment).

source/ci_source/providers/GitHubActions.ts

@@ -132,6 +132,8 @@ import { readFileSync, existsSync } from "fs"
  *     env: GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
  * ```
  *
+ * [GitHub automatically creates a `GITHUB_TOKEN` secret to use in your workflow](https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token). You can use the `GITHUB_TOKEN` to authenticate in a workflow run.
+ *
  */
 
 export class GitHubActions implements CISource {