Add README to examples

Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>

Author: wiktor-k

examples/README.md

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+# Agent examples
+
+The examples in this directory show slightly more elaborate use-cases that can be implemented using this crate.
+
+## Agents
+
+### `key-storage`
+
+Implements a simple agent which remembers RSA private keys (added via `ssh-add`) and allows fetching their public keys and signing using three different signing mechanisms.
+
+This example additionally shows how to extract extensions from messages and works on all major OSes.
+
+It is used in integration tests that run as part of the CI.
+
+### `openpgp-card-agent`
+
+Allows using OpenPGP Card devices to sign SSH requests.
+The PIN is stored in memory and can be time-constrained using SSH constraints.
+For the sake of simplicity this agent supports only `ed25519` subkeys.
+
+This example additionally shows how to create custom protocol based on SSH extensions (in this case decrypt/derive feature).
+
+### `agent-socket-info`
+
+Shows how to extract information about the underlying connection.
+For example under Unix systems this displays connecting process PID.
+To keep the example brief the data is printed as part of a fake public key comment.
+
+## Clients
+
+### `pgp-wrapper`
+
+Wraps SSH keys in OpenPGP data thus allowing OpenPGP applications (such as GnuPG) to read and work with SSH keys.
+This makes it possible to create OpenPGP signatures utilizing SSH keys.
+
+If the connecting agent supports derive/decrypt extension this example additionally creates a decryption subkey and can be used to decrypt OpenPGP data.
+
+### `proto-dumper`
+
+A simple forwarding example which works as an agent and client at the same time dumping all messages and forwarding them to the next agent.
+
+### `ssh-agent-client`
+
+Dumps identities stored by the agent.
+Additionally invokes an extension and reads the result.