Documentation: Add man page for criu-ns

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

Documentation/Makefile

@@ -12,6 +12,7 @@ endif
 
 FOOTER		:= footer.txt
 SRC1		+= crit.txt
+SRC1		+= criu-ns.txt
 SRC1		+= compel.txt
 SRC8		+= criu.txt
 SRC		:= $(SRC1) $(SRC8)

Documentation/criu-ns.txt

@@ -0,0 +1,30 @@
+CRIU-NS(1)
+==========
+include::footer.txt[]
+
+NAME
+----
+criu-ns - run criu in different namespaces
+
+SYNOPSIS
+--------
+*criu-ns* 'dump' -t PID [<options>]
+
+*criu-ns* 'pre-dump' -t PID [<options>]
+
+*criu-ns* 'restore' [<options>]
+
+DESCRIPTION
+-----------
+The *criu-ns* command executes 'criu' in a new PID and mount namespace.
+The purpose of this wrapper script is to enable restoring a process tree
+that might require a specific PID that is already used on the system;
+so called "PID mismatch" problem.
+
+SEE ALSO
+--------
+nsenter(1) namespaces(7) criu(8)
+
+AUTHOR
+------
+The CRIU team