Add license for users to be able to use the custom rules

[ruancomelli]

It was correctly pointed out in a comment to my PR that we should add a license to allow users to use the custom rules defined in the Sourcery config file.

Without a license, users are theoretically not allowed to copy-paste-tweak those rules.

We will probably want a license similar to Semgrep's, which allows people to do practically anything they want with the rules as long as they are used with Semgrep's hosted services.