der: add doc example for Decode

[dishmaker]

use der::{Any, Decode, Reader};

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

impl<'a> Decode<'a> for AnySerde {
    type Error = der::Error;

    fn decode<R: Reader<'a>>(reader: &mut R) -> der::Result<Self> {
        // calls impl Decode for Any
        Ok(Self(Any::decode(reader)?))
    }
}