Add environment variables

Author: ben221199

README.md

@@ -16,4 +16,8 @@ To build this module, you just run:
 make build
 ```
 
-Note: Because of Cgo, `gcc` is expected to be installed.
+Note: Because of Cgo, `gcc` is expected to be installed.
+
+## Environment variables
+
+- `PKCS11_SUBMODULE` - An absolute path to the real PKCS#11 module to load.