Added PowerShell instructions

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Added PowerShell-specific instructions for how to override the cf CLI binary name to the README

Changes proposed in this pull request:

• Include instructions for updating the CF_BINARY_NAME environment variable in PowerShell.

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

None