build-scx-selftests: add README

build-scx-selftests/README.md

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+# Build selftests/sched_ext
+
+This action builds selftests/sched_ext given a kernel build
+output. Kernel build configuration is supposed to include necessary
+flags (i.e. `tools/testing/selftests/sched_ext/config`).
+
+The action is expected to be executed by a workflow with access to the
+Linux kernel repository.
+
+## Required inputs
+
+* `kbuild-output` - Path to the kernel build output.
+* `repo-root` - Path to the root of the Linux kernel repository.
+* `arch` - Kernel build architecture.
+* `toolchain` - Toolchain name: `gcc` (default) or `llvm`.
+
+## Optional inputs
+* `llvm-version` - LLVM version, used when `toolchain` is `llvm`. Default: `16`.
+* `max-make-jobs` - Maximum number of jobs to use when running make (e.g argument to -j). Default: 4*nproc.
+