Add README.md

Author: StefanScherer

golang-issue-18555/README.md

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+# Golang issue 18555
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+This is a test for [golang/go#18555](https://github.com/golang/go/issues/18555): cmd/go: build inside a Windows container does not find the sources in mounted volume
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+See if latest master branch fixes this issue. Let's test it on AppVeyor.