Add docs for ALLOWED_URL_SCHEMES (#56) #155

Job	Run time
linux (x86_64)	56s
macos	1m 14s
linux (i686)	1m 24s
linux-cross (aarch64)	3m 43s
musllinux (x86_64-unknown-linux-musl)	52s
linux-cross (armv7)	3m 35s
musllinux (i686-unknown-linux-musl)	52s
musllinux-cross (aarch64-unknown-linux-musl, aarch64)	1m 41s
linux-cross (s390x)	3m 24s
windows (x64)	1m 12s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	1m 24s
linux-cross (ppc64le)	3m 39s
windows (x86)	1m 32s
linux-cross (ppc64)	1m 31s
Release	0s
	26m 59s

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