doc: note the new "common/" subdir to the doc.go file

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

doc.go

@@ -10,6 +10,9 @@ The "rbd" sub-package wraps APIs that handle RBD specific functions.
 
 The "cephfs" sub-package wraps APIs that handle CephFS specific functions.
 
+The "common" sub-package contains sub-packages related to implementing
+common interfaces and utilities shared across the above.
+
 Consult the documentation for each package for additional details.
 */
 package ceph