📖 Update README.md

Add known issues, screenshots and intro sections

README.md

@@ -1,3 +1,13 @@
+<p style="text-align: center;">
+    ![Piano icon](https://raw.github.com/JacoBits/piano/master/images/logos/logo128.png)
+</p>
+
+Piano is a desktop app that uses built-in MIDI support in order to
+offer an usable interface for playing any of the 127 instruments defined
+by General MIDI.
+
+[Download latest release](https://github.com/JacoBits/piano/releases)
+
 # Building
 
 Using Grunt:
@@ -9,6 +19,47 @@ $ grunt build:win   # use build:linux or build:mac for other systems
 
 Then run the app in `build/win` folder.
 
+# Screenshots
+
+<p style="text-align: center;">
+    ![Main window](https://raw.github.com/JacoBits/piano/master/images/screenshots/main.png)
+    *Main window*
+</p>
+
+<p style="text-align: center;">
+    ![With a music loaded](https://raw.github.com/JacoBits/piano/master/images/screenshots/music.png)
+    *With a music loaded*
+</p>
+
+<p style="text-align: center;">
+    ![Instruments window](https://raw.github.com/JacoBits/piano/master/images/screenshots/instruments.png)
+    *Instruments window*
+</p>
+
+# Known issues
+
+## Pre-release
+
+This app is currently in pre-release state, which means it isn't stable
+and that all work you have done might be lost at any time.
+
+If you find any bug or have any improvement idea, please fill an issue.
+
+## Languages
+
+Currently, the app is only available in French. New languages will be
+added soon.
+
+## MIDI support
+
+The app won't produce any sound on systems that doesn't have at least one
+bundled MIDI output. Maybe should we ship with a default synthetiser.
+
+## Linux/Mac
+
+The app has not yet been tested on Linux and Mac. If you encounter any
+problem, please fill an issue.
+
 # Credits
 
 Icons made by: