Update README.

[monochromegane]

Thank you for providing such a useful tool.
When I was installing it, I found some parts of the README that needed updating, so I created a pull request.

permissions

The permissions given to the GITHUB_TOKEN for the job should be minimal.
In some articles, they choose the “Read and write permissions” button in the “Workflow permissions” section under “Settings > Actions > General”.
However, we can use permissions to modify the default permissions granted to the GITHUB_TOKEN.
ref: https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#permissions

Additional setting

After setting permissions, there is one more setting required to run tagpr.
For this reason, I propose adding this setting to the README.

Here is final minimum setting for running tagpr:

actions/checkout

To prevent the following warning, I propose using newer version.
The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v3.

[Songmu]

Thank you!