Update timeline in 2026/README.md (#1221)

This both updates it based on selection-process.md updates and adds a link to selection-process.md for the further breakdown.

Author: gsnedders

2026/README.md

@@ -4,10 +4,10 @@ Interop 2026 is an effort to increase interoperability across browsers in key te
 
 ## Timeline
 
-**Interop 2026 opened for proposals on September 4, 2025** and the overall timeline is as below. Please note that the timelines are subject to change.
-- Proposal submission window (~3 weeks): Sept 4th, 2025 through Sept 24th, 2025
-- Proposal selection: Before December 11th 2025
-- Scope of the Interop 2026 project published: the first half of February, 2026.
+**Interop 2026 closed for proposals on September 24, 2025**; the overall timeline is as below. Please note that the timelines are subject to change.
+- Proposal submission window (~3 weeks): September 4, 2025 through September 24, 2025.
+- Proposal selection: September 24, 2025 through December 18, 2025. (See the [proposal selection process](./selection-process.md) for breakdown of substeps here.)
+- Scope of the Interop 2026 project published: first half of February, 2026.
 
 ## Guidance for proposal authors