Added Unit documentation

unit/README-short.txt

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+Official build of NGINX Unit: a polyglot app server, a reverse proxy, and a static file server

unit/content.md

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+# What is Unit?
+
+NGINX Unit is a polyglot app server, a reverse proxy, and a static file server, available for UNIX-like systems. It was built by nginx team members from scratch to be highly efficient and fully configurable at runtime. It is licensed under the Apache 2.0 license. The supported application languages are Go, JavaScript, Java, Perl, PHP, Python, Ruby, and more.
+
+> [Key Features](https://unit.nginx.org/#key-features)
+
+%%LOGO%%
+
+# Available tags
+
+Please see [Unit's website](http://unit.nginx.org/installation/#docker-images) for details.
+
+# How to use this image
+
+Please check the [Initial Configuration](http://unit.nginx.org/installation/#initial-configuration) section on Unit's website. Additionally, our [Docker howto](http://unit.nginx.org/howto/docker/) can be of help.

unit/get-help.md

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+the [GitHub issues page](https://github.com/nginx/unit/issues), the [NGINX community](https://nginxcommunity.slack.com/)

unit/github-repo

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+https://github.com/nginx/unit

unit/license.md

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+View [license information](https://raw.githubusercontent.com/nginx/unit/master/LICENSE) for the software contained in this image.

unit/maintainer.md

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+[the Unit Docker Maintainers](%%GITHUB-REPO%%)