der: docs: add Encode trait example

[dishmaker]

use der::{Any, Encode, Length, Reader, Writer};

/// Wrapper around Any, with custom foreign trait support.
///
/// For example: serde Serialize/Deserialize
pub struct AnySerde(pub Any);

impl Encode for AnySerde {

    fn encoded_len(&self) -> der::Result<Length> {
        self.0.encoded_len()
    }

    fn encode(&self, encoder: &mut impl Writer) -> der::Result<()> {
        self.0.encode(encoder)
    }
}