docs: structure intro

Author: hynek

README.md

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    <strong>Please consider <a href="https://github.com/sponsors/hynek">joining them</a> to help make <em>structlog</em>’s maintenance more sustainable!</strong>
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+## Introduction
 
 *structlog* has been successfully used in production at every scale since **2013**, while embracing cutting-edge technologies like *asyncio*, context variables, or type hints as they emerged.
 Its paradigms proved influential enough to [help design](https://twitter.com/sirupsen/status/638330548361019392) structured logging [packages across ecosystems](https://github.com/sirupsen/logrus).