selftests: riscv: Add README for RISC-V KSelfTest

selftests: riscv: Add README for RISC-V KSelfTest

Add a README file for RISC-V specific kernel selftests under
tools/testing/selftests/riscv/. This mirrors the existing README
for arm64, providing clear guidance on how the tests are architecture
specific and skipped on non-riscv systems. It also includes
standard make commands for building, running and installing the
tests, along with a reference to general kselftest documentation.

Signed-off-by: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20250815180724.14459-1-reddybalavignesh9979@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>