Add emoji legend to build meeting issues (#127)

For consistency with other meetings, add the section explaining the emoji's to react with to indicate whether invitees are intending to attend the meeting to the build meeting issues.

templates/meeting_base_build

@@ -7,4 +7,16 @@ JOINING_INSTRUCTIONS="
 
 * link for participants: <https://zoom.us/j/715960833>
 * For those who just want to watch: We stream our conference call straight to YouTube so anyone can listen to it live, it should start playing at <https://www.youtube.com/c/nodejs+foundation/live> when we turn it on. There's usually a short cat-herding time at the start of the meeting and then occasionally we have some quick private business to attend to before we can start recording & streaming. So be patient and it should show up.
-* youtube admin page: <https://www.youtube.com/my_live_events?filter=scheduled>"
+* youtube admin page: <https://www.youtube.com/my_live_events?filter=scheduled>
+
+---
+
+**Invitees**
+
+Please use the following emoji reactions in this post to indicate your
+availability.
+
+* :+1: - Attending
+* :-1: - Not attending
+* :confused: - Not sure
+"