Updated Readme for retention of credit card code
Not using changes in:
https://github.com/simpleinvoices/simpleinvoices/commit/8631709085cb4b50bc7d29ca362ec54c93fbf2a5
https://github.com/simpleinvoices/simpleinvoices/commit/7a0b1d2d9a9caf38869f81da139b94950434e3f5

Also such changes will otherwise break proposed merge of API webservices functionality in Issue #167
https://github.com/simpleinvoices/simpleinvoices/issues/167
https://github.com/mariapetrova/simpleinvoices/commit/c8cf574c33f4d7a17fb8ad4c89bcf6ec79c5e813