Add comment describing SkColorTable constructor.

Review URL: https://codereview.chromium.org/1011263002

include/core/SkColorTable.h

@@ -26,6 +26,8 @@ class SK_API SkColorTable : public SkRefCnt {
 public:
     SK_DECLARE_INST_COUNT(SkColorTable)
 
+    /** Copy up to 256 colors into a new SkColorTable.
+     */
     SkColorTable(const SkPMColor colors[], int count);
     virtual ~SkColorTable();