Explain abscense of OpenSSL in README

[Emilgardis]

Readme should mention why OpenSSL is not in image, provide solutions (i.e rustsl), and provide a workaround by requiring user to maintain an image with OpenSSL-dev installed.

Related:
#565

[0xf1e]

Yes please 🥺

I'm setting up my first pipeline at the moment, and as a basic test I thought I'd have my pipeline compile cargo.
cargo happens to require OpenSSL, and I spent the past days thinking I was doing some fundamental mistake regarding dependencies.

As far as I understood, compiling cargo is considered a very basic test, and cross not passing this test with its default setup should be explained somewhere, in my opinion.

[Alexhuszagh]

This is documented now in the wiki, at least, for how to create an image with OpenSSL installed or how to use the vendored OpenSSL. Using external dependencies is also documented, however, I believe the wiki should be referenced from the README and some of the duplicated information should be removed from the README.

[Alexhuszagh]

Documented on the wiki.

[Emilgardis]

The reason why it's not included is not mentioned though, I feel like we could still do it, but thats minor I think

[Alexhuszagh]

I feel like it was explained: that it was the source of numerous bugs while maintaining images.

Why isn't OpenSSL installed? Maintaining images with OpenSSL proved to be a source of numerous bugs (see #229 and #332), and only some images provide OpenSSL to begin with.

I did fail the hotlinks, so I've restored those.

[Emilgardis]

I meant in the readme, that description is good though!