Mention that build-wheels.sh needs execute permissions

[Safihre]

Although this is probably very straightforward for experienced Travis users, but as a newby on Windows it took me a really long time to figure out that my builds were failing because build-wheels.sh needs execute permissions.
As a Windows user, the only way to do that is from a Linux system..

docker: Error response from daemon: invalid header field value "oci runtime error: container_linux.go:247: starting container process caused \"exec: \\\"/io/travis/build-wheels.sh\\\": permission denied\"\n".

[nanonyme]

@Safihre this is not quite true. Bash in Git for Windows allows you to set executable flag using
git update-index --chmod=+x myfile
Should be mentioned in documentation regardless