default security file

**Is your feature request related to a problem? Please describe.**

Most plugins do not offer any form of security guarantee. Most authors probably haven't considered adding one. A default file in their repo would force authors to address this.

**Describe the solution you'd like**

Each plugin either

a. explicitly offers a security guarantee
b. explicitly **does not** offer a security guarantee

Adding a default to this template would make authors consider their position

**Describe alternatives you've considered**

Increase awareness of `asdf` consumers of their security responsibilities/due diligence.

**Additional context**

The issue names no file, test, or entry point; start by reviewing the asdf-vm/asdf-plugin-template repository to determine where a default security file belongs and what wording the template should provide. Done means generated plugins clearly state whether their authors offer a security guarantee.

Written by the indexing model from the issue text.