[docs]: document how to build the default validator

Signed-off-by: Dmitry Balashov <[email protected]>

default_validator/README.md

@@ -0,0 +1,8 @@
+# `iroha_default_validator`
+
+Use `iroha_wasm_builder_cli` in order to build it:
+
+```bash
+cargo run --bin iroha_wasm_builder_cli -- \
+  build ./default_validator --optimize --outfile ./configs/peer/validator.wasm
+```