docs: add mailbox_sem example Makefile

examples/mailbox_sem/Makefile

@@ -0,0 +1,22 @@
+# Build/run mailbox + semaphore smoke test against this tree's install.
+
+ROOT      ?= $(abspath ../..)
+PREFIX    ?= $(ROOT)/install
+IVERILOG  ?= $(PREFIX)/bin/iverilog
+VVP       ?= $(PREFIX)/bin/vvp
+VVP_MOD   ?= $(PREFIX)/lib/ivl
+
+OUT := mbx_sem_basic.vvp
+
+.PHONY: all run clean
+
+all: $(OUT)
+
+$(OUT): mbx_sem_basic.sv
+	$(IVERILOG) -g2012 -o $@ $<
+
+run: $(OUT)
+	$(VVP) -M $(VVP_MOD) $(OUT)
+
+clean:
+	rm -f $(OUT)