docs(readme): mention the optional parallel feature

Add the rayon feature to the install snippet alongside `serde` so
users discover it from the README.

README.md

@@ -15,8 +15,11 @@ framework concepts.
 [dependencies]
 heuropt = "0.1"
 
-# Optional: derive serde::{Serialize, Deserialize} on the core data types.
+# Optional features:
-# heuropt = { version = "0.1", features = ["serde"] }
+# - "serde":     derive Serialize/Deserialize on the core data types.
+# - "parallel":  evaluate populations across rayon's thread pool.
+#                Seeded runs stay bit-identical to serial mode.
+# heuropt = { version = "0.1", features = ["serde", "parallel"] }
 ```
 
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