docs: add-to-project requires read permissions on the issues (#12)

Unfortunately
[`actions/add-to-project`](https://github.com/actions/add-to-project)
does not support repos/projects under different organisations. I should
probably add an issue.

README.md

@@ -98,7 +98,7 @@ The fine-grained `GITHUB_PAT` secret must include the following permissions:
 | Permission | Access |
 | --- | --- |
 | Projects | Read and write |
+| Pull requests | Read |
 
-Critically, the token does not need access to the repository itself. If the
-repository and the project board are in different organizations, the token only
-needs permissions to the latter.
+Note that fine-grained access tokens cannot grant permissions to projects and repositories in different organisations simultaneously.
+In those cases, you will need an unrestricted _classical_ github token instead.