Add FPGA project workspace layout

Software/kernel/README.md

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+# Software/kernel/
+
+Kernels that run on the core. Starts as a tiny custom kernel for learning
+trap/scheduling/syscalls; later hosts the Linux device tree, OpenSBI, and
+mainline Linux build artifacts.
+
+**Phases:** 11 (tiny kernel), 19 (Linux boot contract), 20 (Linux).