Add more setup info to `check-pr` docs
# PR integration test tool
-Requirements:
+## Requirements
-* Python packages (See `requirements.txt`)
+* Python >= 3.7, and dependencies (see `requirements.txt`)
* [az-cli](https://docs.microsoft.com/en-us/cli/azure/install-azure-cli).
* [azcopy](https://docs.microsoft.com/en-us/azure/storage/common/storage-use-azcopy-v10)
* [azure-functions-core-tools](https://docs.microsoft.com/en-us/azure/azure-functions/functions-run-local?tabs=v3%2Clinux%2Ccsharp%2Cportal%2Cbash%2Ckeda#install-the-azure-functions-core-tools)
+
+## Setup
+
+1. In GitHub, generate a personal access token (PAT) with the `public_repo` scope.
+ You may need to enable SSO for the token, depending on the org that your OneFuzz fork belongs to.
+1. Set `GITHUB_ISSUE_TOKEN` equal to the above PAT in your shell session.
+ [`direnv`](https://direnv.net/) can help here.
+1. Create and activate a virtualenv, e.g. `python -m venv venv` followed by `. ./venv/bin/activate` (e.g. on Linux).
+1. Install dependencies with `pip install -r requirements.txt`
+
+You can now invoke `check-pr.py`.
+See `./check-pr.py -h` for available commands.
