Add basic deny.toml file

[davidpdrsn]

Is your feature request related to a problem? Please describe.
We often use cargo-deny for rust projects but this template doesn't contain a default deny.toml so we're likely to forget adding one, like I did with mirror-mirror.

Describe the solution you'd like
Add a default deny.toml. Could be EmbarkStudios/mirror-mirror#58

[repi]

you could just add it? that one looks reasonable.