Add presentation for EH to support two-phase unwinding

This is a presentation that summarizes
WebAssembly/exception-handling#123. I think the discussion is likely to
take long, so I'm not sure how long I should book for this.

main/2020/CG-08-18.md

@@ -26,6 +26,9 @@ Installation is required, see the calendar invite.
 1. Proposals and discussions
     1. Review of action items from prior meeting.
     1. A Meta-meta-meta Request for Proposals (7.5 minutes)
+    1. Proposal for changes in EH proposal for two-phase unwinding support +
+       discussions (Heejin Ahn) [25 min]
+       - Poll: ?
 1. Closure
 
 ## Agenda items for future meetings