Add AppImage build instructions

This PR add instructions for producing an AppImage on each build. It's built using appiamge-builder and can be used in almost every modern GNU/Linux distribution. It require some additional tuning from your side in order to properly set the version.

Please review and let me know your comments.

Providing an AppImage would have, among others, these advantages:

Applications packaged as an AppImage can run on many distributions (including Ubuntu, Fedora, openSUSE, CentOS, elementaryOS, Linux Mint, and others)
One app = one file = super simple for users: just download one AppImage file, make it executable, and run
No unpacking or installation necessary
No root needed
No system libraries changed
Works out of the box, no installation of runtimes needed
Optional desktop integration with appimaged
Optional binary delta updates, e.g., for continuous builds (only download the binary diff) using AppImageUpdate
Can optionally GPG2-sign your AppImages (inside the file)
Works on Live ISOs
Can use the same AppImages when dual-booting multiple distributions
Can be listed in the AppImageHub central directory of available AppImages
Can double as a self-extracting compressed archive with the --appimage-extract parameter
No repositories needed. Suitable/optimized for air-gapped (offline) machines
Decentralized