Added Orange-Textable-Prototypes to add-on list

[axanthos]

No description provided.

[lanzagar]

I think we should work on improving the guidelines and policy of the list of Orange addons visible from the in-application dialog (i.e. make it clearer in this repository's README?).

Currently, it is not intended to be an exhaustive list, but should feature the addons we consider stable enough and of interest to the everyday user.
I am not sure we want to have prototypes of addons in this list (Orange prototypes is an exception and usually contains widgets that are considered for core Orange and of general interest). I think the addon dialogue/list should contain the main and stable addon (e.g. Textable). If the developers or an involved user want to install the prototypes, they can easily do so using the More button and pasting in the package name.

As this is the first issue of this sort, I will not close the PR yet, and leave it open if anyone has some additional comments or other views. But so far, my opinion is that addon prototypes should not be on the list.
@markotoplak @janezd

[stuart-cls]

For orange-spectroscopy-prototypes, I've have had no issues getting testers to use the "Add more..." button.