Add build instruction for Linux and setup AppImage builds

Please, add instruction for build Astree on Linux.

Also, lets setup Travis CI for auto-build AppImage (portable package for Linux) for each commit:

- https://github.com/appimage/AppImageKit
- https://github.com/probonopd/linuxdeployqt
- https://github.com/linuxdeploy/linuxdeploy-plugin-qt
- https://github.com/probonopd/uploadtool

For example, take a look on LeoCAD `.travis.yml` implementation:

- https://github.com/leozide/leocad/blob/master/.travis.yml

And here is what resulted builds would look like

- https://github.com/leozide/leocad/releases/tag/continuous

P.S.: I really like *Astree* app idea! 😉 And would be happy to test it as, by myself, has needs to design few optical devices 📷 🔬 🔭