add readme

README.md

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+# Crypto Helper
+
+Helper functions for cryptography
+
+## Usage
+
+```js
+const crypto = require('@scode/crypto-helper');
+
+const rand_hex = crypto.random_hex(16); // outputs 16 byte random hex
+const rand_salt = crypto.create_salt(); // same as random_hex, but with fixed length of 32 bytes
+const random_string = crypto.random_string(16) // output 16 character long random string
+const hash = crypto.hash_sha512(random_string, random_hex); // returns sha 512 hex
+const check = crypto.checksum('foo'); // returns a sha 256 hex
+
+// jwt like object signing
+const signed = crypto.sign_object({foo: 'bar'}, 'secret');
+const dec = crypto.decode_signed(signed); // decode a signed object without verifying the signature
+const ver = crypto.verify_signature(signed, 'secret', 10000); // verifies the signature and returns the contents. the timeout is in milliseconds and optional, timing will be ignored if omitted.
+```