Add Configurations repo info to config/README.md (#19583)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

config/README.md

@@ -1,3 +1,9 @@
 # Where have all the configurations gone?
 
+Marlin configurations for specific machines are now maintained in their own repository at:
+
+## https://github.com/MarlinFirmware/Configurations
+
+Configuration files for use with the nightly `bugfix-2.0.x` branch can be downloaded from:
+
 ## https://github.com/MarlinFirmware/Configurations/archive/bugfix-2.0.x.zip