Add documentation for explaining proxy usage during staging
Add documentation for explaining proxy usage during staging

See cloudfoundry/docs-buildpacks#52

[#116609747]

Signed-off-by: Dave Goddard <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@goddard.id.au>

cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book,cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book,swisscom/docs-appcloud-book,swisscom/docs-appcloud-book