Document what is known about the need to specify a COMPENSATION parameter.

Signed-off-by: Brent Nelson <nelson@ee.byu.edu>

xc/xc7/techmap/cells_map.v

diff --git a/xc/xc7/techmap/cells_map.v b/xc/xc7/techmap/cells_map.v
index fd93e82..51554c7 100644
--- a/xc/xc7/techmap/cells_map.v
+++ b/xc/xc7/techmap/cells_map.v
@@ -4790,6 +4790,16 @@
 
   parameter BANDWIDTH = "OPTIMIZED";
   parameter STARTUP_WAIT = "FALSE";
+
+// Previously, the default COMPENSATION value was ZHOLD, resulting in non-functional 
+//   bitstreams when the feedback loop is closed on-chip.
+  // Setting it to INTERNAL as the default creates working bitstreams for that case.
+// This bug was not previously uncovered since the MMCM tests all explicitly
+//   specified a COMPENSATION value so the ZHOLD default was never used.
+// Setting it here in the techmapper means that existing code using on-chip 
+//   MMCM feedback without specifying a COMPENSATION value can be ported 
+//   unmodified into the toolflow and will result in functional bitstreams.
+// A test was added to test the case when relying on the default COMPENSATION value.
   parameter COMPENSATION = "INTERNAL";
 
   parameter CLKIN1_PERIOD = 0.0;