add README

README.md

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+<div align="center">
+
+  <h1>PoW JavaScript library</h1>
+
+<strong>JavaScript library to generate PoW for mCaptcha</strong>
+
+[![0.1.0](https://img.shields.io/badge/TypeScript_docs-master-2b7489)](https://mcaptcha.github.io/pow_sha256-polyfill/)
+![Build)](<https://github.com/mCaptcha/pow_sha256-polyfill/workflows/CI%20(Linux)/badge.svg>)
+[![codecov](https://codecov.io/gh/mCaptcha/pow_sha256-polyfill/branch/master/graph/badge.svg)](https://codecov.io/gh/mCaptcha/pow_sha256-polyfill)
+
+</div>
+
+## Usage
+
+To generate proof-of-work, per mCaptcha specification:
+
+```typescript
+import {generate_proof} from "@mcaptcha/pow_sha256-polyfill"; /* yet to
+be published in npm */
+
+
+let salt = "randomsaltvalueprovidedbymcaptcha";
+let phrase = "randomphrasevalueprovidedbymcaptcha";
+let difficulty = 50_000;
+
+let work = await generate_proof(salt, phrase, difficulty);
+```