Add a README.md to rustls-cert-gen

rustls-cert-gen/README.md

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+# rustls-cert-gen
+
+`rustls-cert-gen` is a tool to generate TLS certificates. In its
+current state it will generate a Root CA and an end-entity
+certificate, along with private keys. The end-entity certificate will
+be signed by the Root CA.
+
+## Usage
+Having compiled the binary you can simply pass a path to output
+generated files.
+
+	cargo run -- -o output/dir
+
+In the output directory you will find these files:
+
+  * `cert.pem`  (end-entity's X.509 certificate, signed by `root-ca`'s key)
+  * `cert.key.pem` (end-entity's private key)
+  * `root-ca.pem` (ca's self-signed x.509 certificate)
+
+For a complete list of supported options:
+
+	rustls-cert-gen --help
+
+## FAQ
+
+#### What signature schemes are available?
+
+  * `pkcs_ecdsa_p256_sha256`
+  * `pkcs_ecdsa_p384_sha384`
+  * `pkcs_ed25519`