Add note about LDC

Author: LunaTheFoxgirl

README.md

@@ -1,3 +1,8 @@
 # Objective-D
 
-Objective-D provides essential bindings to Objective-C APIs.
+Objective-D provides essential bindings to Objective-C APIs.
+
+## NOTE
+
+This relies on a D compiler which supports generating Objective-C compatible objects.
+Eg. DMD for x86_64 macs, or LDC with [This PR](https://github.com/ldc-developers/ldc/pull/4777).