tar-artifacts: add action README

tar-artifacts/README.md

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+# Tar build artifacts
+
+This action creates a tarball with kbuild-output and other build
+artifacts necessary to run the selftests.
+
+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`.
+
+# Outputs
+
+* `archive-name` - full path to zstd-compressed tarball with the artifacts
+
+# Archive options
+
+Essential content of the directory passed via `kbuild-output` input is
+always included in the tarball.
+
+For selftests artifacts the script checks environment variables to
+determine what to include. These are handled as bash flags:
+emptystring means false, any other value means true.
+
+* `ARCHIVE_BPF_SELFTESTS` - add `tools/testing/selftests/bpf` binaries
+  under `selftests/bpf` in the tarball
+* `ARCHIVE_MAKE_HELPERS` - add all the Linux repo makefiles and other
+  scripts
+* `ARCHIVE_SCHED_EXT_SELFTESTS` - add
+  `tools/testing/selftests/sched_ext` binaries under
+  `selftests/sched_ext` in the tarball
+
+
+