Add initial in memoriam text for @gkellogg.

Author: BigBlueHat

ack-script/acknowledgements.html

@@ -1,5 +1,16 @@
 <section id="ack" class="appendix informative">
     <h2>Acknowledgements</h2>
+
+    <h3>In Memoriam</h3>
+    <p>Gregg Kellogg was a central figure in the story of JSON-LD. He worked
+    tirelessly on JSON-LD specifications until his death on September 6th, 2025.
+    Gregg’s passion for solving hard problems with finesse was only exceeded by
+    his willingness to work with others on arriving at that destination. The
+    JSON-LD community will be forever grateful for Gregg’s consistent, careful,
+    and kindly delivered contributions throughout JSON-LD’s most formative
+    days.</p>
+
+    <h3>Contributions</h3>
     <p>
         The editors would like to specially thank the following individuals for making significant
         contributions to the authoring and editing of this specification: