Add comment to wasm_lib.rs

src/wasm_lib.rs

@@ -1,3 +1,11 @@
 #![allow(special_module_name)]
 
 mod lib;
+
+# This is an hack, but currently there are no ways to change
+# the crate-type conditionally, and Wasm extensions and native
+# extensions need two different defaults.
+
+# This file sole purpose is remapping the content of lib as an
+# example, so that it can be built via:
+# cargo built --example package_name