Update README.md

[jmikedupont2]

please mention at top level dir

[ShoyuVanilla]

Well, I don't think we should inform this in README. The binary that installed with cargo xtask is not for "retails":

rust-analyzer/xtask/src/install.rs

Lines 138 to 145 in e9968fc

	fn install_server(sh: &Shell, opts: ServerOpt) -> anyhow::Result<()> {
	let features = opts.malloc.to_features();
	let profile = if opts.dev_rel { "dev-rel" } else { "release" };
	let mut install_cmd = cmd!(
	sh,
	"cargo install --path crates/rust-analyzer --profile={profile} --locked --force --features force-always-assert {features...}"
	);

That enables force-always-assert, which turns on panicking on many internal assertions that just leaving error logs in our "retail" build. So, this might cause irritating panics to uses, so we cannot recommend that by default.
Also, the manual page already contains various ways to install rust-analyzer and it's not hidden so deep inside the nested chapters, so I think current manual page link is enough

[lnicola]

Agreed, let's link to https://rust-analyzer.github.io/book/rust_analyzer_binary.html instead (even though there's another link to the manual right above, maybe change that to https://rust-analyzer.github.io/book/installation.html.

And the force-always-assert thing isn't actually documented in the manual.