Create default gitignore file

[jrgmartin]

When the project is first created and before the first commit is done, please create a gitignore file with the following contents:

robots.txt
/.htaccess
wp-config.php
wp-content/advanced-cache.php
wp-content/db.php
wp-content/object-cache.php
wp-content/cache/
wp-content/upgrade/
wp-content/w3tc-config/