Is it possible to add a text input within the product entity?

I would like to be able to add a text input field on the Add product page on the admin, this input would hold a string as data and I was going to save in a plugins table. The thing is that in this link it says that the input should be readable only. Does anyone knows to tell me why? If this is a solution, how could I fetch this on my back end, lets say in a ProductSubscriber?

Thanks in advance for any help!