Document workarounds for installing Gephi on Linux

Originally posted by @cschwem2er at https://github.com/gephi/gephi/issues/1417:

> As several users report crashes when using Linux (https://github.com/gephi/gephi/issues1192) and a temporary fix was suggested by @3ruce, it would be very nice to have a short tutorial for two things:
>
> 1) How to add Gephi as a PPA repository, so that linux (ubuntu/ mint) users always get the latest version
> 2) How to enable options like "LIBGL_ALWAYS_SOFTWARE=1" without having to start Gephi via the terminal each time.
>

首先查看 issue 请求以及 Gephi 网站现有的安装文档。记录 Ubuntu 或 Mint 用户如何添加 Gephi PPA，以及如何在不从终端启动 Gephi 的情况下启用 LIBGL_ALWAYS_SOFTWARE=1；当这两种解决方法都在一篇简短、实用的教程中得到说明时，即视为完成。

由索引模型根据 Issue 内容生成。