Openzeppelin-contracts: SPDX license identifiers missing

[wbt]

As of today (Solidity 0.6.8, from this PR), Solidity issues warnings on all OpenZeppelin (and other) contracts for lack of an "SPDX-License-Identfier" code comment at the top of the file.

A proposed fix would be to add
//SPDX-License-Identifier: MIT
to the start of every contract file.

[wbt]

Any chance of putting out a patch release with this in it?

[nventuro]

That's a great suggestion @wbt, thanks! I just published v3.0.2 with that change, you should be able to install it from npm now and not get any warnings.