Add AppImageBuilder

[lulol]

Builds an AppImage executable package with the mlnet+gui executable. https://github.com/ygrek/mldonkey/pull/65#issuecomment-1105902001

git clone https://github.com/ygrek/mldonkey.git
cd mldonkey
./configure --enable-batch --disable-directconnect --disable-fasttrack --disable-gnutella --disable-gnutella2 --enable-gui=newgui2 --prefix=/usr
make

wget https://github.com/AppImageCrafters/appimage-builder/releases/download/v1.0.3/appimage-builder-1.0.3-x86_64.AppImage
chmod +x appimage-builder-1.0.3-x86_64.AppImage
ln -s packages/appimage/AppImageBuilder.yml AppImageBuilder.yml
./appimage-builder-1.0.3-x86_64.AppImage

To run the optional appimage-builder tests you need docker installed and permissions.