[os_must_gather] Add new variable in README

Commit 6cd893b added new variable
called cifmw_os_must_gather_volume_percentage, adding it on the
README.

Signed-off-by: Arnau Verdaguer <[email protected]>

Author: averdagu

roles/os_must_gather/README.md

@@ -15,6 +15,7 @@ testing the new changes.
 * `cifmw_os_must_gather_host_network`: (Bool) Flag to gather host network data
 * `cifmw_os_must_gather_namespaces`: (List) List of namespaces required by the gather task in case of failure
 * `cifmw_os_must_gather_additional_namespaces`: (String) List of comma separated additional namespaces. Defaults to `kuttl,openshift-storage,sushy-emulator`
+* `cifmw_os_must_gather_volume_percentage`: (int) Specify maximum percentage of must-gather pod's allocated volume that can be used. If this limit is exceeded, must-gather will stop gathering, but still copy gathered data. Defaults to `70`
 
 ## Examples
 ```