contrib/cmd: add README.md to clarify the intent

So as to clarify that distro maintainers should not include these
contrib command binaries (`recvtty`, `sd-helper`, `seccompagent`, etc.)
in their packages.

ref: https://bugs.launchpad.net/ubuntu/+source/runc/+bug/2076981

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Makefile

@@ -79,6 +79,8 @@ runc-bin: runc-dmz
 
 .PHONY: all
 all: runc recvtty sd-helper seccompagent fs-idmap memfd-bind pidfd-kill remap-rootfs
+	@echo "*Note*: Only the runc binary should be installed to user environments."
+	@echo "Other binaries are provided as examples or test utilities. Users do not need them."
 
 .PHONY: recvtty sd-helper seccompagent fs-idmap memfd-bind pidfd-kill remap-rootfs
 recvtty sd-helper seccompagent fs-idmap memfd-bind pidfd-kill remap-rootfs:

contrib/cmd/README.md

@@ -0,0 +1,3 @@
+The command packages here are provided as either examples or test utilities.
+
+These packages are not expected to be installed to user environments.