pfSense

Update `Submitting a Pull Request via GitHub` link

Signed-off-by: Josh Soref <2119212+jsoref@users.noreply.github.com>

Spelling fixes. Fix #13357

• Incorrect input validation for `dhcp6c` `keyinfo expire` `forever` keyword in `interfaces.inc`.
• Incorrect input validation for associated rule's `Source Port` in `firewall_rules_edit.php`.
• Incorrect `voucher*.` file lock reference in `status_captiveportal_voucher_rolls.php`....

Add new CONTRIBUTING file and Pull Request template