docs: add readme for justfile/pr-checks update

pr-checks/readme.md

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+# PR Checks
+
+This folder contains the code supporting the workflows run when a PR is created.
+
+## Update
+
+If you need to make a change to any of the PR checks, you need to perform the following
+steps:
+
+1. Make the change - the code for the PR checks is under the `pr-checks/checks/` folder.
+2. Run the `sync.py` file to produce (and sync) the final workflow files under `.github/`
+
+The second part requires some associated steps (create a virtual environment, download
+the dependencies for the Python script, etc), so we have automated this with the `justfile`
+included in this folder.
+
+### 1-step update
+
+1. Install https://github.com/casey/just by whichever way you prefer.
+2. Run `$ just update-pr-checks` in your terminal.
+
+If you don't wish to install `just`, you can also manually perform the steps
+outlined in the `justfile` under the `update-pr-checks` action.