Update installation instructions

[noah-de]

Otherwise new comers who want to follow along would see something like this:
'go get' is no longer supported outside a module.
To build and install a command, use 'go install' with a version,
like 'go install example.com/cmd@latest'
For more information, see https://golang.org/doc/go-get-install-deprecation
or run 'go help get' or 'go help install'.

[mna]

go install with the version specifier is to install commands (executables). Goquery is a package, go get is the right way to install it, it's just that you now have to be in a Go module to install it. I'd welcome a PR that makes that clearer.

[noah-de]

Thanks for the kind reply.
I am still getting used to how things work in Go, (recognizing a project's purpose via the structure of the source code).

Now I see that Goquery is a package installed (as a source based dependency) and then imported into code.
The documentation does a pretty good job of showing that.
I was aware of the official discussion, golang/go#40276 but am still new enough to the language that I misunderstood the meaning, until now.

Many thanks!