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 <[e-mail]>
Reviewed-by: Andrew Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paul Walmsley <[e-mail]>

tools/testing/selftests/riscv/README

···
+KSelfTest RISC-V
+================
+
+- These tests are riscv specific and so not built or run but just skipped
+  completely when env-variable ARCH is found to be different than 'riscv'.
+
+- Holding true the above, RISC-V KSFT tests can be run within the
+  KSelfTest framework using standard Linux top-level-makefile targets:
+
+      $ make TARGETS=riscv kselftest-clean
+      $ make TARGETS=riscv kselftest
+
+      or
+
+      $ make -C tools/testing/selftests TARGETS=riscv \
+		INSTALL_PATH=<your-installation-path> install
+
+      or, alternatively, only specific riscv/ subtargets can be picked:
+
+      $ make -C tools/testing/selftests TARGETS=riscv RISCV_SUBTARGETS="mm vector" \
+		INSTALL_PATH=<your-installation-path> install
+
+   Further details on building and running KSFT can be found in:
+     Documentation/dev-tools/kselftest.rst