README.md: mention Marble-Mini

Since the QR URL on that first batch actually points here

Author: ldoolitt

README.md

@@ -43,3 +43,12 @@ In the KiCad (version 5.1.x) schematic GUI (eeschema):
 
 ![layout](marble_top.png)
 
+## Special note about the prototype Marble-Mini
+
+Due to evolving branding ideas, the first prototype run of
+[Marble-Mini](https://github.com/BerkeleyLab/Marble-Mini) boards was
+marked "Marble v1.0rc3" and the QR code pointed to this Marble repo.
+That's a *different board*: 4xSFP instead of 2xQSFP, and no SO-DIMM socket
+are the most obvious indicators.
+You can go [directly](https://github.com/BerkeleyLab/Marble-Mini/releases/tag/v1.0rc3)
+to the documentation related to that manufacturing run.