dm: add documentation for IMA measurement support

To interpret various DM target measurement data in IMA logs,
a separate documentation page is needed under
Documentation/admin-guide/device-mapper.

Add documentation to help system administrators and attestation
client/server component owners to interpret the measurement
data generated by various DM targets, on various device/table state
changes.
Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Documentation/admin-guide/device-mapper/index.rst

@@ -13,6 +13,7 @@ Device Mapper
     dm-dust
     dm-ebs
     dm-flakey
+    dm-ima
     dm-init
     dm-integrity
     dm-io