Added documentation for packed pragma

doc/manual.txt

@@ -5390,6 +5390,11 @@ checked.
 **Future directions**: GC'ed memory should be allowed in unions and the GC
 should scan unions conservatively.
 
+Packed pragma
+-------------
+The `packed`:idx: pragma can be applied to any ``object`` type. It ensures that the fields of an object is packed back-to-back in memory. It is useful to store packets or messages from/to network or hardware drivers, and for interoperability with C. Combining packed pragma with inheritance is not defined, and it should not be used with GC'ed memory (ref's).  
+
+**Future directions**: Using GC'ed memory in packed pragma will result in compile-time error. Usage with inheretinence should be defined and documented.
 
 Unchecked pragma
 ----------------

web/news.txt

@@ -61,6 +61,7 @@ News
       evaluation.
     - ``--gc:none`` produces warnings when code uses the GC.
     - A ``union`` pragma for better C interoperability is now supported.
+    - A ``packed`` pragma to control the memory packing/alignment of fields in an object.
     - Arrays can be annotated to be ``unchecked`` for easier low level
       manipulations of memory.