Add calendar item for 2021 edition tracking

Author: Mark-Simulacrum

text/0000-roadmap-2020.md

@@ -292,6 +292,8 @@ in 2020, versus the timing from this year.
 * June
   * Publish progress report, describing what work we have done so
     far towards the goals of this roadmap
+  * Identify more fine-grained deadlines for 2021 edition work. Schedule
+    internal meetings in teams for July to verify progress.
 * July
 * August
   * Start organizing 2020 Rust survey