Add Withdrawn proposals to README.md (#120)

Author: marcoscaceres

Diff for: README.md

@@ -7,7 +7,6 @@ This repo is a place where WebKit contributors may post [explainers](https://git
 * [Centralized Consent API](CentralizedConsentAPI/)
 * [Controlling Animation Frame Rate](animation-frame-rate/)
 * [Coordinated media playback through MediaSession](MediaSessionCoordinator/)
-* [Cryptographic Message Syntax (CMS) API](https://github.com/WebKit/explainers/blob/main/cryptographic-message-syntax-API/README.md)
 * [Declarative Web Push](DeclarativeWebPush/)
 * ["remote" CryptoKeys](remote-cryptokeys/)
 * [TextTrackCue enhancements for programmatic subtitle and caption presentation](texttracks/)
@@ -24,3 +23,12 @@ Proposals which have moved on to incubation or standardization in another venue.
 | [`<model>` element](https://github.com/immersive-web/model-element) | [Immersive Web CG](https://www.w3.org/community/immersive-web/) |
 | [IsLoggedIn](https://github.com/privacycg/is-logged-in) | [Privacy CG](https://github.com/privacycg) |
 | [Private Ad Measurement](https://github.com/patcg-individual-drafts/private-ad-measurement) | [Private Advertising Technology CG](https://patcg.github.io/) |
+
+
+## Withdrawn proposals
+
+Propoosals that we decided to no longer pursue. 
+
+| Proposal | Reason | Date Withdrawn |
+| --------- | ----------- |----------| 
+|[Cryptographic Message Syntax (CMS) API](https://github.com/WebKit/explainers/blob/main/cryptographic-message-syntax-API/README.md) | After discussions with industry colleagues, it was decided a different approach might be more appropriate (e.g., a JS or WASM library) | 2024-06-04 |