Point hardware/ at the new KiCad hardware repository

The earlier Eagle files stay where they are so existing links and forks keep
resolving; v3 onwards lives in Protocentral/protocentral_pulse_express_hardware.
This repository remains the home of the Arduino library.

Author: awhitchurch

hardware/README.md

@@ -0,0 +1,24 @@
+# Hardware — earlier revision (archived)
+
+The Eagle design files in this folder (`pc_pulse Express.sch`,
+`pc_pulse Express.brd`, `pc_pulse express_schematic.pdf`) are an earlier
+revision of the ProtoCentral Pulse Express board.
+
+They are kept here permanently so that existing links, forks, and references to
+these files continue to work. They are no longer the current design.
+
+## Current design files
+
+Revision **v3** and everything that follows it live in a dedicated hardware
+repository, as a KiCad project:
+
+**https://github.com/Protocentral/protocentral_pulse_express_hardware**
+
+| | |
+|---|---|
+| v3 schematic (PDF) | [protocentral-pulse-express-v3-schematic.pdf](https://github.com/Protocentral/protocentral_pulse_express_hardware/blob/main/hardware/protocentral-pulse-express-v3-schematic.pdf) |
+| v3 KiCad project | [`hardware/`](https://github.com/Protocentral/protocentral_pulse_express_hardware/tree/main/hardware) |
+| Product page | https://protocentral.com/product/pulse-express-pulse-ox-heart-rate-sensor-with-max32664/ |
+
+This repository remains the home of the **Arduino library** for Pulse Express.
+Only the hardware design files have moved.