Create appveyor.yml

Copy of yml from https://github.com/ocaml/opam/blob/master/appveyor.yml which I will edit to do mono build
[CI] Update appveyor.yml

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
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Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>