docs: add README to soroban-test (#549)

cmd/crates/soroban-test/README.md

@@ -0,0 +1,54 @@
+Soroban Test
+============
+
+Test framework wrapping Soroban CLI.
+
+Provides a way to run tests aganist a local sandbox; running against RPC endpoint _coming soon_.
+
+
+Overview
+========
+
+- `TestEnv` is a test environment for running tests isolated from each other.
+- `TestEnv::with_default` invokes a closure, which is passed a reference to a random `TestEnv`.
+- `TestEnv::new_assert_cmd` creates an `assert_cmd::Command` for a given subcommand and sets the current
+   directory to be the same as `TestEnv`.
+- `TestEnv::cmd` is a generic function which parses a command from a string.
+   Note, however, that it uses `shlex` to tokenize the string. This can cause issues
+   for commands which contain strings with `"`s. For example, `{"hello": "world"}` becomes
+   `{hello:world}`. For that reason it's recommended to use `TestEnv::cmd_arr` instead.
+- `TestEnv::cmd_arr` is a generic function which takes an array of `&str` which is passed directly to clap.
+   This is the preferred way since it ensures no string parsing footguns.
+- `TestEnv::invoke` a convenience function for using the invoke command.
+
+
+Example
+=======
+
+```rs
+use soroban_test::{TestEnv, Wasm};
+
+const WASM: &Wasm = &Wasm::Release("soroban_hello_world_contract");
+const FRIEND: &str = "friend";
+
+#[test]
+fn invoke() {
+    TestEnv::with_default(|workspace| {
+        assert_eq!(
+            format!("[\"Hello\",\"{FRIEND}\"]"),
+            workspace
+                .invoke(&[
+                    "--id",
+                    "1",
+                    "--wasm",
+                    &WASM.path().to_string_lossy(),
+                    "--",
+                    "hello",
+                    "--to",
+                    FRIEND,
+                ])
+                .unwrap()
+        );
+    });
+}
+```