Update 09-R_Core_Developers.Rmd

R-core: new since 3 days ago: Sebastian Meyer; no longer Martin Morgan

Author: mmaechler

09-R_Core_Developers.Rmd

@@ -14,7 +14,8 @@ This page lists the former and current members of the R Core team who have write
   * Kurt Hornik (present)
   * Luke Tierney (present)
   * Martin Maechler (present)
-  * Martin Morgan (present)
+  * Sebastian Meyer (present)
+  * Martin Morgan (up to June 2021)
   * Martyn Plummer (present)
   * Michael Lawrence (present)
   * Paul Murrell (present)