Updates readme with billing_project mount option.
@@ -110,6 +110,7 @@ with dashes instead of underscores:
* `limit_bytes_per_sec`
* `stat_cache_ttl`
* `type_cache_ttl`
+* `billing_project`
On both OS X and Linux, you can also add entries to your `/etc/fstab` file like
the following:
