Add a README.md to `test/BaselineOutput`

We should have a README.md to the test/BaselineOutput folder to explain its purpose.

(Updated on 2018-07-02 to reflect the new location of the folder)

[costin-eseanu]

Because it would be sorted last in the folder view, so it’s easy to find. 😃

[TomFinley]

@justinormont I like your /test/BaselineOutput suggestion.

The intention of that directory is that it mirrors the TestOutput directory, produced by our tests. (In this way we can review any divergences between the expected output (in the baselines), and the actual output (as actually output by the test.

This however brings up what might be a larger (different?) issue. The test outputs used to be written just to the root directory, alongside ZBaselines. I might like them to be written to /test/TestOutput… that way, you get the two peer directories right next to each other.

Right now, as near as I can tell, the test output is not being written to a single directory, which makes tracking changes to baselines harder to find, and makes the practice of using diff tools to see all the test divergences at a glance now impossible (I have, for example, one TestOutput directory at Microsoft.ML.Core.Tests\netcoreapp2.0\TestOutput in my build, and another TestOutput directory at ``). Also it’s being put in a very, very odd location: the bin directory right next to the test assemblies. I’m not sure the binary directory is really an appropriate location for test output.

I’m not sure why it was changed to this new system exactly, but we should revisit, maybe instead writing them to test/TestOutput, or something.

Also hi @costin-eseanu , you seem familiar somehow. 😄