add readme (#2)

README.md

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+# ReCap
+
+A Typescript implementation of [ERC-5573 ReCap](https://eips.ethereum.org/EIPS/eip-5573), enabling applications to express [UCAN-style object capabilities](https://github.com/ucan-wg/spec) using [Sign In With Ethereum](https://eips.ethereum.org/EIPS/eip-4361).
+
+This package is still ALPHA software, bugs may exist and APIs may change in future.
+
+## Installation
+
+ReCap can be installed via `npm`:
+
+``` sh
+npm -i siwe-recap
+```
+
+`yarn`:
+
+``` sh
+yarn add siwe-recap
+```
+
+or `package.json` entry:
+
+``` json
+{
+  "dependencies": {
+    "siwe-recap": "0.1.0"
+  }
+}
+```
+
+## Usage
+
+ReCaps are designed to be used in conjunction with SIWE. They can be initialized and built as follows:
+
+``` typescript
+import { Recap } from 'siwe-recap';
+
+const recap = new Recap();
+
+recap.addAttenuation('https://example.com/my/resource', 'crud', 'read');
+recap.addAttenuation('https://example.com/my/resource', 'crud', 'update', { maxTimes: 5 });
+recap.addProof('bagaaierasords4njcts6vs7qvdjfcvgnume4hqohf65zsfguprqphs3icwea');
+
+console.log(recap.attenuations);
+// {
+//   "https://example.com/my/resource": {
+//     "crud/read": [{}],
+//     "crud/update": [{ maxTimes: 5 }]
+//   }
+// }
+```
+
+They can also be applied to or extracted from SIWE messages:
+
+``` typescript
+import { SiweMessage } from 'siwe';
+import { Recap } from 'siwe-recap';
+
+const siwe = new SiweMessage( ... );
+
+// we can extract the recap while verifying that the statement in `siwe` matches
+// the recap listed in `siwe`s resource list
+const recap = Recap.extract_and_verify(siwe);
+
+// 
+recap.addAttenuation('https://example.com/my/resource', 'crud', 'read');
+
+// the new change will be merged into the existing recap in `siwe`
+recap.add_to_siwe_messate(siwe);
+```