add license file so it can be found by go-licenses

[bollwyvl]

Thanks for this! I'm trying to package go-ipfs for conda-forge, and this is one of the few modules that doesn't have a discoverable license file.

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[rvagg]

as discussed in https://github.com/ipld/go-car/pull/46 I think these PRs should be for both LICENSE-MIT & LICENSE-APACHE.

[Stebalien]

This project has non-PL contributors, so it'll have to go through the relicensing process. For now, I'm just merging the MIT license because the README has always stated that the license was "MIT".