Clarify Windows Build Instructions

[mhils]

The "install failed" output on Windows links to a README section which does not exist:

rust-openssl/openssl-sys/build/find_normal.rs

Lines 179 to 184 in 0138f52

	It looks like you're compiling for MSVC but we couldn't detect an OpenSSL
	installation. If there isn't one installed then you can try the rust-openssl
	README for more information about how to download precompiled binaries of
	OpenSSL:
	https://github.com/sfackler/rust-openssl#windows

https://docs.rs/openssl/latest/openssl/ hints that vcpkg is one of the recommended options, and indeed running

vcpkg install --triplet=x64-windows-static-md openssl

does work as expected (although I first had to figure out that I need to supply --triplet).

I'm happy to send a PR to update either error message or add a README section if someone lets me know what would be preferred. :)

[timzaak]

Both is ok, but I would try to read the error message firstly, and then read readme and search windows in this project issue.

[indirection42]

Vote for it🙋. This issue really help me a lot. Before that, I have been searched the SO and installed other packages.