[Github File] Add note about clashing URLs, rel #82 (#90)

Co-authored-by: Lea Verou <[email protected]>

backends/github/file/README.md

@@ -13,6 +13,14 @@ Write & read data and upload files in GitHub repositories.
 
 Note that if the branch name is not in the URL, Madata will try `main`, and then `master`.
 
+<div class=note>
+
+The simplified URLs like `https://github.com/:owner/:repo/:path` could clash with the ones supported by the [GitHub Labels](../labels/) backend in the (unlikely) case the path starts from a directory called `issues`, `labels`, or `milestones`.
+
+To avoid that, you can use one of the other URL formats or construct the backend object directly from the `GithubFile` class: `new GithubFile("https://github.com/leaverou/health-data/milestones/2024/labels").`
+
+</div>
+
 ## Constructor options
 
 * `allowForking`: Whether to fork on save if logged in user has no permission to push. Defaults to `false`.