feat: Default env variables

README.md

@@ -185,6 +185,27 @@ let command = cmd!(
 assert_eq!(format!("{command:?}"), r#"NEW_VAR="new_var" TEST="test" "echo" "test""#);
 ```
 
+#### Conditional
+You can have a default value set for an environment variable.
+
+```rust
+use comlexr::cmd;
+
+const NEW_VAR: &str = "NEW_VAR";
+std::env::set_var("TEST", "realvalue");
+
+let command = cmd!(
+    env {
+        "TEST":? "test",
+        NEW_VAR: "new_var"
+    };
+    "echo",
+    "test",
+);
+
+assert_eq!(format!("{command:?}"), r#"NEW_VAR="new_var" "echo" "test""#);
+```
+
 #### Current Directory and Env Variable Order Matters
 Environment variable declarations **MUST** come after the current directory declaration.