add readme to siwe package (spruceid#65)

packages/siwe/README.md

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+![Sign-In with Ethereum logo](https://login.xyz/favicon.png "Sign-In with Ethereum logo")
+
+Sign-In with Ethereum describes how Ethereum accounts authenticate with 
+off-chain services by signing a standard message format parameterized by scope,
+session details, and security mechanisms (e.g., a nonce). The goals of this 
+specification are to provide a self-custodied alternative to centralized 
+identity providers, improve interoperability across off-chain services for 
+Ethereum-based authentication, and provide wallet vendors a consistent 
+machine-readable message format to achieve improved user experiences and 
+consent management.
+
+## Quickstart Examples
+To try it out locally, check out these examples:
+
+- [Node](https://github.com/spruceid/siwe-quickstart/tree/main/00_print)
+- [Frontend](https://github.com/spruceid/siwe-quickstart/tree/main/01_frontend)
+- [Backend](https://github.com/spruceid/siwe-quickstart/tree/main/02_backend)
+- [End to end](https://github.com/spruceid/siwe-quickstart/tree/main/03_complete_app)
+- [Sign-In with Ethereum Notepad](https://github.com/spruceid/siwe-notepad)
+
+## Motivation
+When signing in to popular non-blockchain services today, users will typically 
+use identity providers (IdPs) that are centralized entities with ultimate 
+control over users' identifiers, for example, large internet companies and email
+providers. Incentives are often misaligned between these parties. Sign-In with
+Ethereum offers a new self-custodial option for users who wish to assume more
+control and responsibility over their own digital identity.
+
+Already, many services support workflows to authenticate Ethereum accounts using
+message signing, such as to establish a cookie-based web session which can 
+manage privileged metadata about the authenticating address. This is an 
+opportunity to standardize the sign-in workflow and improve interoperability 
+across existing services, while also providing wallet vendors a reliable method 
+to identify signing requests as Sign-In with Ethereum requests for improved UX.
+
+This work is sponsored by the Ethereum Foundation and Ethereum Name Service 
+(ENS). It is being developed in the open through a series of recorded community 
+calls and public repositories, and its development is informed by over twenty 
+user interviews with a focus on currently-in-production uses, related prior 
+EIPs, and fits within product roadmaps.
+
+## Specification
+Specification can be found [here](https://eips.ethereum.org/EIPS/eip-4361).
+
+## Disclaimer 
+
+Our TypeScript library for Sign-In with Ethereum has not yet undergone a formal security 
+audit. We welcome continued feedback on the usability, architecture, and security 
+of this implementation.
+
+## Mono Repo Install and Build
+Run `npm install` to install dependencies, then `npm bootstrap` to link the dependencies
+in their respective packages. Afteward run `npm run build` to build the library.
+Development can occur on the `package/*` level with tests being run on each package itself.
+