Provide some installation documentation

[schmittjoh]

For non-symfony users, it might be beneficial to provide some installation instructions.

see #207

[tsi]

@kasperisager, Thanks, your project helped me very much.
For anyone interested - basic installation is quite simple (composer is used to install a single symfony component - Process) :

1. Clone/Download the project
2. in your project directory, run curl -s https://getcomposer.org/installer | php
3. and then - php composer.phar install

Assetic is now ready to use, check out the docs directory or @kasperisager's project for examples.

[Evan-R]

I managed to get it working without any command line; I'll fork and put up a demo when I get some time