Add a note about tables being auto-generated (#3208)

* Add a note about tables being auto-generated

* Update README.md

Co-authored-by: Pablo Baeyens <pbaeyens31+github@gmail.com>

* spell

---------

Co-authored-by: Pablo Baeyens <pbaeyens31+github@gmail.com>

Author: cijothomas

.cspell.yaml

@@ -238,6 +238,7 @@ words:
   - Quesma
   - raesene
   - reiley
+  - readmemd
   - roadmaps
   - rolldice
   - ruech

README.md

@@ -125,6 +125,7 @@ SIG-specific GitHub discussions.
 > The meeting times in the tables below are given in 24-hour notation.
 > Meetings are either in Pacific Time (PT), with Daylight Saving Time, or UTC+8, without Daylight Saving Time.
 
+<!-- The tables below are auto-generated. To make changes, see CONTRIBUTING.md#updating-sig-information-on-the-readmemd -->
 <!-- sigs -->
 ### Specification SIGs