Add warning text to documentation on `templateUrl` parameter in tsp init command

[markcowl]

External templates can contain malicious packages and the like. We prompt about this when folks use tsp init:

warning: Downloading or using an untrusted template may contain malicious packages that can compromise your system and data. Proceed with caution and verify the source.
? Continue (Y/n)

We should also add a similar warning to the reference doc on tsp init