readme: added appimagehub link (#798)

Author: stone-w4tch3r

README.md

@@ -99,6 +99,7 @@ For **macOS** users:
 For **Linux** users:
 
 * `DEB` or `RPM` packages are available at [buildkite](https://buildkite.com/organizations/sourcegit/packages).
+* `Appimage` files can be found on [AppimageHub](https://appimage.github.io/SourceGit/)
 * `xdg-open` must be installed to support open native file manager.
 * Make sure [git-credential-manager](https://github.com/git-ecosystem/git-credential-manager/releases) is installed on your linux.
 * Maybe you need to set environment variable `AVALONIA_SCREEN_SCALE_FACTORS`. See https://github.com/AvaloniaUI/Avalonia/wiki/Configuring-X11-per-monitor-DPI.