describe the installation process

I started to describe the installation process.

Tasks:

- [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;
- [x] Describe how to finish the installation with the web ui;
- [x] Describe how to finish the installation using the ``install`` command;
- [x] Delete the orphaned cookbook entry;
- [x] Load custom migrations;
- [x] Load custom fixtures;
- [x] Install other versions than ``1.0.0``;
- [x] Explain configuration parameters;
- [x] Describe why node.js is recommended and how to install it;
- [x] Explain the ``--force`` option.