Warn when creating a contract importing from @openzeppelin/contracts

[abcoathup]

Warn the user when creating a contract importing from @openzeppelin/contracts with a reference to the documentation: https://docs.openzeppelin.com/sdk/2.5/linking#linking-the-contracts-ethereum-package

(We may need to move the Note about using @openzeppelin/contracts-ethereum-package` so that it is visible when viewing the link)

So the warning could be something like the following:

oz create
...
Contract Test imports `@openzeppelin/contracts`. Change to `@openzeppelin/contracts-ethereum-package`. 
See https://docs.openzeppelin.com/sdk/2.5/linking#linking-the-contracts-ethereum-package.

Similar to the warning for an explicit constructor:

Contract Test has an explicit constructor. Change it to an initializer function. 
See https://docs.openzeppelin.com/sdk/2.5/writing_contracts.html#initializers.