README: document errors in xbps-uunshare(8)/uchroot(8).

README.md

@@ -28,6 +28,9 @@ This utility requires these linux kernel options:
 - CONFIG\_UTS\_NS
 - CONFIG\_USER\_NS
 
+This is the default method, and if your system does not support any of the required kernel
+options it will fail with `EINVAL (Invalid argument)`.
+
 #### unshare(1)
 
 This utility also requires `user_namespaces(7)` support and these linux kernel options:
@@ -65,6 +68,10 @@ To enable it:
     $ cd void-packages
     $ echo XBPS_CHROOT_CMD=uchroot >> etc/conf
 
+If for some reason it's erroring out as `ERROR clone (Operation not permited)`, check that
+your user is a member of the required `group` and that `xbps-uchroot(8)` utility has the
+proper permissions and owner/group as explained above.
+
 #### proot(1)
 
 The `proot(1)` utility implements chroot and bind mounts support completely in user space,