Add self-hosted execution example
#
# Running WASI verison with different engines:
# cp wasm3.wasm ./
+# ./run-spec-test.py --exec "../build/wasm3 wasm3.wasm --repl"
# ./run-spec-test.py --exec "wasmtime --dir=. wasm3.wasm -- --repl"
# ./run-spec-test.py --exec "wasmer run --dir=. wasm3.wasm -- --repl"
# ./run-spec-test.py --exec "wasmer run --dir=. --backend=llvm wasm3.wasm -- --repl"
