Updates to installation instructions

I'm proposing a few additions and clarifications based on my experience installing openDAM:

 * The cache/ and log/ directories do not exist when cloning the git repository (since they are empty) so they may need to be created.

 * Apache's mod_rewrite must be installed and enabled for the application to work.

 * Comment out the PHP version note in the config sample.  (I didn't notice it there, and copied the line directly to my php.ini, and then got a confusing syntax error upon reload.)

 * A few clarifications about the database creation and the initial user account that is created (including reminding the user to change that account's password or delete it)

 * Finish translating a few phrases to English. :-)