Add warning for mul_add feature

Author: youknowone

Cargo.toml

@@ -7,7 +7,11 @@ license = "PSF-2.0"
 
 
 [features]
-# Turning on this feature on aarch64-apple-darwin helps bit representation compatibility
+# Do not enable this feature unless you really need it.
+# CPython didn't intend to use FMA for its math library.
+# This project uses this feature in CI to verify the code doesn't have additional bugs on aarch64-apple-darwin.
+# However, this does not mean that using this feature is better or more correct on that platform.
+# Note that the quality of the results has been well-tested without FMA but has not been tested with FMA.
 # See also: https://github.com/python/cpython/issues/132763
 mul_add = []