README: fixup troubleshooting docs based on #742 (#759)

Signed-off-by: William Woodruff <[email protected]>

README.md

@@ -468,9 +468,10 @@ ordinary `pip`:
   [`pip`'s documentation](https://pip.pypa.io/en/stable/topics/authentication/#using-keyring-as-a-command-line-application)
   explains these in depth.
 
-In practice, this means that authenticated third-party indices that **don't** take
-a username, like Google Artifact Registry, are currently **unsupported** by `pip-audit`.
-See [#742](https://github.com/pypa/pip-audit/issues/742) for more details.
+In addition to the above, some third-party indices have required, hard-coded usernames.
+For example, for Google Artifact registry, the hard-coded username is `oauth2accesstoken`.
+See [#742](https://github.com/pypa/pip-audit/issues/742) and
+[pip#11971](https://github.com/pypa/pip/issues/11971) for additional context.
 
 ## Tips and Tricks