example/Nexus_K3P_S: Add example design for Cisco Nexus K35-S/K3P-S

Signed-off-by: Alex Forencich <alex@alexforencich.com>

example/Nexus_K3P_S/fpga/README.md

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+# Taxi Example Design for Nexus K35-S/K3P-S (ExaNIC X10/X25)
+
+## Introduction
+
+This example design targets the Cisco Nexus K35-S/K3P-S (ExaNIC X10/X25) FPGA board.
+
+The design places looped-back MACs on the SFP+ cages.
+
+* SFP+ cages
+  * Looped-back 10GBASE-R or 25GBASE-R MAC via GTH or GTY transceiver
+
+## Board details
+
+* FPGA:
+  * K35-S/X10: xcku035-fbva676-2-e
+  * K3P-S/X235: xcku3p-ffvb676-2-e
+* 25GBASE-R PHY: Soft PCS with GTH or GTY transceiver
+
+## Licensing
+
+* Toolchain
+  * Vivado Standard (enterprise license not required)
+* IP
+  * No licensed vendor IP or 3rd party IP
+
+## How to build
+
+Run `make` in the appropriate `fpga*` subdirectory to build the bitstream.  Ensure that the Xilinx Vivado toolchain components are in PATH.
+
+## How to test
+
+Run `make program` to program the board with Vivado.
+
+To test the looped-back MAC, it is recommended to use a network tester like the Viavi T-BERD 5800 that supports basic layer 2 tests with a loopback.  Do not connect the looped-back MAC to a network as the reflected packets may cause problems.