copy license text from: tidyverse/haven
The existing HAVEN_LICENSE file was lacking any form of actual license.
After checking the pandas source, it appears that the reference to HAVEN
is in relation to tidyverse/haven, copying the license from
tidyverse/haven for clarity.

Signed-off-by: Jamin Collins <jamin@amazon.com>

For reference, can you dig up the commit in haven adding their license? When we added the test files, Haven apparently didn't include a license yet: https://github.com/pandas-dev/pandas/pull/26537#issuecomment-496671885 Looks like the license was added as a result of this PR:
https://github.com/tidyverse/haven/pull/484

and this commit:
https://github.com/tidyverse/haven/commit/b2410adc3b6428c549452e7b4015e436a31fb4d5 The PR to add here predates the change to add the license, so I don't think should be retro-fitting that here, no? It was noted in this comment https://github.com/pandas-dev/pandas/pull/26537#issuecomment-496671885, that the project (haven) appeared to be under MIT.  The next comment https://github.com/pandas-dev/pandas/pull/26537#issuecomment-496678901 indicates that (hadley) is fine with that.

The PR that I reference https://github.com/tidyverse/haven/pull/484 was merged by what appears to be the same individual.

However, the files in this project do no reflect the MIT license.  This is simply an attempt to clarify what was (apparently) already recorded separately. this is fine. cc @TomAugspurger  Yep, LGTM. I just wanted the verification that we didn’t fail to copy the license initially. 

> On Mar 16, 2020, at 21:46, Jeff Reback <notifications@github.com> wrote:
> 
> ﻿
> this is fine. cc @TomAugspurger
> 
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 Thanks @jamin-aws-ospo !