Add documentation (#110)

docs/index.md

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+# Opengist
+
+Opengist is a **self-hosted** pastebin **powered by Git**. All snippets are stored in a Git repository and can be
+read and/or modified using standard Git commands, or with the web interface. 
+It is similiar to [GitHub Gist](https://gist.github.com/), but open-source and could be self-hosted.
+
+Written in [Go](https://go.dev), Opengist aims to be fast and easy to deploy.
+
+
+## Features
+
+* Create public, unlisted or private snippets
+* [Init](/docs/usage/init-via-git.md) / Clone / Pull / Push snippets **via Git** over HTTP or SSH
+* Revisions history
+* Syntax highlighting ; markdown & CSV support
+* Like / Fork snippets
+* Search for snippets ; browse users snippets, likes and forks
+* Editor with indentation mode & size ; drag and drop files
+* Download raw files or as a ZIP archive
+* OAuth2 login with GitHub, Gitea, and OpenID Connect
+* Avatars via Gravatar or OAuth2 providers
+* Light/Dark mode
+* Responsive UI
+* Enable or disable signups
+* Restrict or unrestrict snippets visibility to anonymous users
+* Admin panel : 
+  * delete users/gists; 
+  * clean database/filesystem by syncing gists
+  * run `git gc` for all repositories
+* SQLite database
+* Logging
+* Docker support
+
+
+## System requirements
+
+[Git](https://git-scm.com/download) is obviously required to run Opengist, as it's the main feature of the app.
+Version **2.20** or later is recommended as the app has not been tested with older Git versions.
+
+[OpenSSH](https://www.openssh.com/) suite if you wish to use Git over SSH
+
+
+## Components
+
+* Backend Web Framework: [Echo](https://echo.labstack.com/)
+* ORM: [GORM](https://gorm.io/)
+* Frontend libraries:
+  * [Tailwind CSS](https://tailwindcss.com/)
+  * [CodeMirror](https://codemirror.net/)
+  * [Day.js](https://day.js.org/)
+  * [highlight.js](https://highlightjs.org/)
+  * and [others](/package.json)