Update README.md

README.md

 SkyAPM PHP
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 <img src="https://skyapmtest.github.io/page-resources/SkyAPM/skyapm.png" alt="Sky Walking logo" height="90px" align="right" />
 
 **SkyAPM PHP** is the PHP instrumentation agent, which is compatible with [Apache SkyWalking](https://github.com/apache/skywalking) backend and others compatible agents/SDKs
 
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 ## Documents
 * [Documents in English](docs/README.md)
 * [中文文档](docs/README_ZH.md)