[pivotal] Instruct users to use a dedicated Shortcut API token

If a user of this importer configures it with a token that is
simultaneously making requests to Shortcut from a different
script/automation/integration, then that significantly increases the
likelihood that they could be throttled by the Shortcut API. This
script does not currently provide explicit handling for HTTP 429
requests, and it would be difficult for users to reason about why they
were being throttled, so I've added this to the documentation at
Toby's suggestion.

Co-authored-by: Toby Crawley <[email protected]>

pivotal-import/README.md

@@ -8,7 +8,7 @@ In order to run this, you will require a Pivotal account and the ability to sign
 
 1. Sign up for a Shortcut account at [https://www.shortcut.com/signup](https://www.shortcut.com/signup).
    - **NOTE:** Do not run this importer against an existing Shortcut workspace that already has data you wish to keep.
-1. [Create an API token](https://app.shortcut.com/settings/account/api-tokens) and [export it into your environment](../Authentication.md).
+1. [Create a new API token](https://app.shortcut.com/settings/account/api-tokens) and [export it into your environment](../Authentication.md). Ensure you use this token only for this importer, so that you aren't rate limited unexpectedly by the Shortcut API.
 1. Export your Pivotal project to CSV and save the file to `data/pivotal_export.csv`.
 1. Create/Invite all users you want to reference into your Shortcut workspace.
    - **NOTE:** If you're not on a Shortcut trial, please [reach out to our support team](https://help.shortcut.com/hc/en-us/requests/new) before running this import to make sure you're not billed for users that you want to be disabled after import.