der: docs: add EncodeValue trait example + doc fn decode_value

[dishmaker]

use der::{Encode, EncodeValue, ErrorKind, FixedTag, Length, Tag, Writer};

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

impl EncodeValue for MyByteMonth {

    fn value_len(&self) -> der::Result<Length> {
        Ok(Length::new(1))
    }

    fn encode_value(&self, writer: &mut impl Writer) -> der::Result<()> {
        writer.write_byte(self.0)?;
        Ok(())
    }
}

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

let month = MyByteMonth(9);
let mut buf = [0u8; 16];
let month_der = month.encode_to_slice(&mut buf).expect("month to encode");

assert_eq!(month_der, b"\x04\x01\x09");

Contains Note from rusticata/asn1-rs#106 in decode_value