Make it clear that this is not suitable for forked PRs (#140)

* Make it clear that this is not suitable for forked PRs

* docs: Revise usage note

---------

Co-authored-by: Ryan Christian <rchristian@ryanchristian.dev>

Author: eoghanmurray

README.md

@@ -29,6 +29,8 @@ jobs:
       - uses: preactjs/compressed-size-action@v2
 ```
 
+> **Note:** Due to GitHub's permission model, this action cannot safely create comments when it is triggered by a PR from a fork. It will, however, still generate the size comparison and print the comment it would've posted to the stdout of the action, allowing manual checking and you can copy/paste it into a comment if you wish.
+
 ### Customizing the Installation
 
 By default, `compressed-size-action` will install dependencies according to which lockfiles are present, if any. However, if you need to run a different installation command, you can pass a custom script to do so. For example, to use `npm ci` with the `--workspace` option: