Add README.md for kelvin.

Change-Id: Ice8cac210ba6b681815a9d3c1342033ff195a32a

README.md

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+# Kelvin
+
+Kelvin is a RISC-V32IM core with a custom instruction set.
+
+## Building
+
+Kelvin uses [bazel](https://bazel.build/) as it's build system. The Verilated
+simulator for Kelvin can be generated using:
+
+```bash
+bazel build //tests/verilator_sim:core_sim
+```
+
+The verilog source for the Kelvin core can be generated using:
+
+```bash
+bazel build //hdl/chisel:core_cc_library_emit_verilog
+```