`.tox` folder should maybe contain a `CACHEDIR.TAG`

[akx]

What's the problem this feature will solve?

Since .tox/ is an ephemeral folder, it could be useful if it contained a CACHEDIR.TAG file.

These files are useful markers that e.g. GNU tar supports to exclude archiving/backing up/... recreatable data.

Describe the solution you'd like

When Tox creates .tox, it adds a spec-compliant CACHEDIR.TAG file there.

[gaborbernat]

PR welcome.