doc/README: add link to CC-license for source code comments

README.md

@@ -2,6 +2,13 @@
 
 An interpreter for the RV32I instruction set.
 
+## License
+
+ISC License
+
+The source code includes quotations from the RISC-V Instruction Set Manual under
+the [Creative Commons Attribution 4.0 International License](https://github.com/riscv/riscv-isa-manual/blob/riscv-user-2.2/LICENSE).
+
 ## References
 
 - RISC-V Instruction Set Manual: https://github.com/riscv/riscv-isa-manual