[Documentation] - It is not very clear which platforms are supported.

[pedropaulosuzuki]

As noted by #181 and denoland/deno#3438, Windows 7 and earlier are no longer supported, as well as any i386 builds of Deno, which is fine. What makes things a little bit more confusing is that it is not very clear that that's the case. I'm a Deno teacher, and some students have not been able to install the software, and they didn't know that it was because the systems they were using were no longer supported. I think a comment about the matter on the compatibility section of the README.md file would solve the issue. Thanks!!