Update readme with workdir parameter

README.md

@@ -22,6 +22,19 @@ This Github action enables you to take the screenshots and videos generated by C
 
 **Required** Slack channels to upload to
 
+### `workdir`
+
+**Optional** The folder where Cypress stores screenshots and videos on the build machine.
+
+Default: `cypress`
+
+(this relative path resolves to `/home/runner/work/<REPO_NAME>/<REPO_NAME>/cypress`)
+
+If your project uses Cypress from the project root folder, the default value will work for you.
+But if your project uses Cypress in a subfolder (like most monorepos), you'll need to provide the relative path to that folder
+(i.e. `e2e/cypress`).
+(Don't include a trailing slash on your path!)
+
 ## Examples
 
 ### Upload results after every push