[DEWIF] precise version format

rfc/0013_Duniter_Encrypted_Wallet_Import_Format.md

@@ -41,6 +41,10 @@ This base64 string encodes a sequence of bytes whose structure is defined below.
 | version (4 bytes) | currency code (4 bytes) | version data (any bytes) |
 |:-----------------:|:-----------------------:|:------------------------:|
 
+## Version
+
+The 4 bytes of the version field must be interpreted as an unsigned integer encoded in big endian.
+
 ## Currencies code
 
 | Currency | code       |