recover old User doc and move installation to readme

docs/README.md

@@ -1,11 +1,55 @@
 # Diffuse documentation
 
-Welcome to Diffuse documentation!
+**Diffuse** is a small and simple text merge tool written in Python.  
 
-Here are the main sections:
+With Diffuse, you can easily merge, edit, and review changes to your code.
+
+- [Quick Start](#quick-start)
+- [Users](users.md)
+- [Translators](translators.md)
+- Developers
+    - [Developers setup](developers/developers-setup.md)
+    - [Release process](developers/release-process.md)
+
+## Installation
+
+### Using Flatpak
+
+```sh
+flatpak install io.github.mightycreak.Diffuse
+```
+
+Flatpak is probably the easiest way to install Diffuse on any distribution and
+get the latest and greatest as quickly as possible. The Flatpak package is
+hosted on [Flathub](https://flathub.org).
+
+_Note:_ If the Flathub repository is not installed yet, please follow
+[these instructions](https://flatpak.org/setup/).
+
+### Using the distribution package manager
+
+The Diffuse package statuses for every distributions can been seen on
+[repology](https://repology.org/project/diffuse/versions).
+
+Huge thanks to [@bongochong](https://github.com/bongochong) for maintaining the
+Diffuse package on Fedora.
+
+## Integrate with Git
+
+Diffuse is compatible with `git difftool` command-line. To use Diffuse as Git
+diff tool, run:
+
+```sh
+git config --global diff.tool diffuse
+```
+
+## Quick Start
+Compare a Set of Files
+ <pre>$ diffuse <em>file1</em> <em>file2</em> <em>file3</em></pre>
+Review Local Changes or Fix Merge Conflicts
+ <pre>$ diffuse -m</pre>
+Compare Specific Revisions
+ <pre>$ diffuse -r <em>rev1</em> -r <em>rev2</em> <em>file</em></pre>
+Inspect a Revision
+ <pre>$ diffuse -c <em>rev</em></pre>
 
-1. [Users](users.md)
-2. [Translators](translators.md)
-3. Developers
-    1. [Developers setup](developers/developers-setup.md)
-    2. [Release process](developers/release-process.md)