Add basic crypto crate README (MystenLabs/narwhal#490)

narwhal/crypto/src/README.md

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+# Signature Schemes and Traits
+
+This crate contains:
+- useful traits that should be implemented by concrete types that represent digital cryptographic 
+material (keys). For signatures, we rely on `signature::Signature`, which may be more widely implemented.
+- concrete implementations of the following signature schemes that implement the recommended traits required for 
+cryptographic agility. The following schemes are implemented (wrappers over existing popular crates):
+    - ed25519 (EdDSA), backed by the [dalek-ed25519](https://github.com/dalek-cryptography/ed25519-dalek) crate (soon 
+to be replaced by the [ed25519-consensus](https://github.com/penumbra-zone/ed25519-consensus) crate).
+    - BLS12-381, backed by the [blst](https://github.com/supranational/blst) crate.
+    - BLS12-377, backed by the [ark_bls12_377](https://docs.rs/ark-bls12-377/0.3.0/ark_bls12_377/) crate. *Note that this
+implementation is under the non-default conditional compilation feature `celo`, and is not compiled or linked without 
+that explicit flag being passed in. The goal of this implementation is to provide an experimental benchmark, it is NOT 
+meant as a production implementation.*
+- An asynchronous [`SignatureService`] (which lives in `lib.rs`) that is instantiated by a `Signer` object.
+
+## Traits
+- [`ToFromBytes`]: this trait aims to minimize the number of steps involved in obtaining a serializable key.
+- [`EncodeDecodeBase64`]: an extension trait of `ToFromBytes` for immediate conversion to/from base64 strings.
+- [`VerifyingKey`]: which denotes associated types for private key and signature material, while it includes a default 
+naive implementation of batch verification (which can be overridden).
+- [`SigningKey`]: associated types for public key and signature material.
+- [`Authenticator`]: associated types for private key and public key material.
+- [`KeyPair`]: which includes the common get priv/pub key functions and a key-pair generation function.
+
+## Tests and Benchmarks
+There exist tests for all the three schemes, which can be run by:  
+```
+$ cargo test
+```
+
+One can compare all currently implemented schemes for *sign, verify, verify_batch* and 
+*key-generation* by running:
+```
+$ cargo bench
+```