Add note on zero-based certificate version value (#790)

Author: stocyr

src/OpenSSL/crypto.py

@@ -1128,7 +1128,8 @@ def from_cryptography(cls, crypto_cert):
 
     def set_version(self, version):
         """
-        Set the version number of the certificate.
+        Set the version number of the certificate. Note that the
+        version value is zero-based, eg. a value of 0 is V1.
 
         :param version: The version number of the certificate.
         :type version: :py:class:`int`