请添加startIndexMethod的使用范例 about jt-framework

[hylexus]

• doc
• sample

    // 经度(尚未除以 10^6)
    @BasicField(startIndex = 12, dataType = DWORD)
    private Integer intLng;
    // 经度(使用转换器除以10^6转为Double类型)
    @BasicField(startIndex = 12, dataType = DWORD, customerDataTypeConverterClass = LngLatReqMsgFieldConverter.class)
    private Double lng;

    // 经度(startIndexMethod使用示例)
    @BasicField(startIndexMethod = "getLngStartIndex", dataType = DWORD, customerDataTypeConverterClass = LngLatReqMsgFieldConverter.class)
    private Double lngByStartIndexMethod;

    // a sample for https://github.com/hylexus/jt-framework/issues/9
    public int getLngStartIndex() {
        log.info("消息体总长度:{}", this.requestMsgMetadata.getHeader().getMsgBodyLength());
        return 12;
    }