[WIP] describe the installation process

I started to describe the installation process.

TODOs:

- [x] Describe how to install Composer;
- [x] Describe how to get the source code via GitHub;
- [x] Describe how to get the source as an archive;
- [x] Describe the installation of the dependencies;
- [x] Describe the database setup;
- [x] Describe the web server setup;
- [ ] Describe how to finish the installation with the web ui;
- [x] Describe how to finish the installation using the ``install`` command.