Document use with the PKCS#8

[XVilka]

Currently, only rsa crate has high-level functions documented that allow to load form PEM, DER, etc formats. But I found nothing of the kind in the elliptic_curve's documentation.

It would be nice to have methods descrubed similarly to those of rsa:

• https://docs.rs/rsa/latest/rsa/#pkcs1-rsa-key-encoding
• https://docs.rs/rsa/latest/rsa/#pkcs8-rsa-key-encoding

There are only basic paragraphs in https://docs.rs/elliptic-curve/latest/elliptic_curve/struct.PublicKey.html#parsing-spki-keys and https://docs.rs/elliptic-curve/latest/elliptic_curve/struct.SecretKey.html#parsing-pkcs8-keys. I myself already figured what to do, but for the newbies it would be nice to have a clearer documentation.

[tarcieri]

It's hard to have concrete code examples because they'd have to be repeated for each elliptic curve, though I suppose we can add some and copy-and-paste them for each curve, but that's a lot of work and makes it hard to ever update them due to the duplication.