Add docs for ALLOWED_URL_SCHEMES #154

Job	Run time
macos	1m 1s
linux (x86_64)	58s
musllinux (x86_64-unknown-linux-musl)	51s
linux-cross (aarch64)	3m 37s
linux (i686)	1m 19s
musllinux (i686-unknown-linux-musl)	1m 3s
musllinux-cross (aarch64-unknown-linux-musl, aarch64)	1m 44s
linux-cross (armv7)	3m 28s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	1m 25s
linux-cross (s390x)	3m 10s
windows (x64)	1m 3s
linux-cross (ppc64le)	3m 16s
windows (x86)	1m 28s
linux-cross (ppc64)	1m 30s
Release	0s
	25m 53s

Provide feedback