der: docs: add DecodeValue trait example

[dishmaker]

use der::{Decode, DecodeValue, ErrorKind, FixedTag, Header, Reader, Tag};

/// 1-byte month
struct MyByteMonth(u8);

impl<'a> DecodeValue<'a> for MyByteMonth {
    type Error = der::Error;

    fn decode_value<R: Reader<'a>>(reader: &mut R, header: Header) -> der::Result<Self> {
        let month = reader.read_byte()?;
        
        if (0..12).contains(&month) {
            Ok(Self(month))
        } else {
            Err(reader.error(ErrorKind::DateTime))
        }
    }
}

impl FixedTag for MyByteMonth {
    const TAG: Tag = Tag::OctetString;
}

let month = MyByteMonth::from_der(b"\x04\x01\x09").expect("month to decode");

assert_eq!(month.0, 9);