add instructions for manual decoding of v2g EXI data

Was able to extract a signed XML payload using the methods added to the readme:

{
  "ns7:V2G_Message": {
    "@xmlns:ns7": "urn:iso:15118:2:2013:MsgDef",
    "@xmlns:xsi": "http://www.w3.org/2001/XMLSchema-instance",
    "@xmlns:ns3": "http://www.w3.org/2001/XMLSchema",
    "@xmlns:ns4": "http://www.w3.org/2000/09/xmldsig#",
    "@xmlns:ns5": "urn:iso:15118:2:2013:MsgBody",
    "@xmlns:ns6": "urn:iso:15118:2:2013:MsgDataTypes",
    "@xmlns:ns8": "urn:iso:15118:2:2013:MsgHeader",
    "ns7:Header": {
      "ns8:SessionID": "81E9D40000000000",
      "ns4:Signature": {
        "ns4:SignedInfo": {
          "ns4:CanonicalizationMethod": {
            "@Algorithm": "http://www.w3.org/TR/canonical-exi/"
          },
          "ns4:SignatureMethod": {
            "@Algorithm": "http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256"
          },
          "ns4:Reference": {
            "@URI": "#ID001",
            "ns4:Transforms": {
              "ns4:Transform": {
                "@Algorithm": "http://www.w3.org/TR/canonical-exi/"
              }
            },
            "ns4:DigestMethod": {
              "@Algorithm": "http://www.w3.org/2001/04/xmlenc#sha256"
            },
            "ns4:DigestValue": "Yf+2hj2l5lI/MOzqhm76FaJeLtgsydIdY/c1lTKNJ8U="
          }
        },
        "ns4:SignatureValue": "HBC9jqiJ9vMm6CmJ62oBZU0wIjAoE7/4SKKzp+toxGhcvGeOZdnVgJQUG4/qQvor66h87FPot03QV/yrJJ0Fxg=="
      }
    },
    "ns7:Body": {
      "ns5:AuthorizationReq": {
        "@ns5:Id": "ID001",
        "ns5:GenChallenge": "KwjKU9ogG9sN+Mk4yuNp/w=="
      }
    }
  }
}

Directly before this packet was one that appears to contain some unknown data which I'm guessing is related to validation of the xmldsig:

<?xml version="1.0" encoding="utf-8"?>
<ns7:V2G_Message xmlns:ns7="urn:iso:15118:2:2013:MsgDef"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:ns3="http://www.w3.org/2001/XMLSchema"
xmlns:ns4="http://www.w3.org/2000/09/xmldsig#"
xmlns:ns5="urn:iso:15118:2:2013:MsgBody"
xmlns:ns6="urn:iso:15118:2:2013:MsgDataTypes"
xmlns:ns8="urn:iso:15118:2:2013:MsgHeader">
  <ns7:Header>
    <ns8:SessionID>81E9D400</ns8:SessionID>
  </ns7:Header>
  <ns7:Body>
    <ns5:PaymentServiceSelectionRes>
      <ns5:ResponseCode>OK</ns5:ResponseCode>
    </ns5:PaymentServiceSelectionRes>
  </ns7:Body>
</ns7:V2G_Message>
line 1 column 206 - Warning: replacing invalid character code 145
line 1 column 207 - Warning: replacing invalid character code 147
<?xml version="1.0" encoding="utf-8"?>
<xsi:nil xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:ns3="http://www.w3.org/2001/XMLSchema"
xmlns:ns4="urn:iso:15118:2:2010:AppProtocol">
&lt;&#225;&#8216;&#8220;/&gt;</xsi:nil>
<?xml version="1.0" encoding="utf-8"?>
<ns7:V2G_Message xmlns:ns7="urn:iso:15118:2:2013:MsgDef"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:ns3="http://www.w3.org/2001/XMLSchema"
xmlns:ns4="http://www.w3.org/2000/09/xmldsig#"
xmlns:ns5="urn:iso:15118:2:2013:MsgBody"
xmlns:ns6="urn:iso:15118:2:2013:MsgDataTypes"
xmlns:ns8="urn:iso:15118:2:2013:MsgHeader">
  <ns7:Header>
    <ns8:SessionID>81E9D400</ns8:SessionID>
  </ns7:Header>
  <ns7:Body>
    <ns5:PaymentDetailsRes>
      <ns5:ResponseCode>OK</ns5:ResponseCode>
      <ns5:GenChallenge>KwjKU9ogG9sN+Mk4yuNp/w==</ns5:GenChallenge>
      <ns5:EVSETimeStamp>0</ns5:EVSETimeStamp>
    </ns5:PaymentDetailsRes>
  </ns7:Body>
</ns7:V2G_Message>

ChargePoint / wireshark-v2g

add instructions for manual decoding of v2g EXI data #5