Update installation instructions

[BoKleynen]

Installing executables using go get is nog longer supported as of go 1.18, from the official release notes:

go get no longer builds or installs packages in module-aware mode. go get is now dedicated to adjusting dependencies in go.mod. Effectively, the -d flag is always enabled. To install the latest version of an executable outside the context of the current module, use go install example.com/cmd@latest. Any version query may be used instead of latest. This form of go install was added in Go 1.16, so projects supporting older versions may need to provide install instructions for both go install and go get. go get now reports an error when used outside a module, since there is no go.mod file to update. In GOPATH mode (with GO111MODULE=off), go get still builds and installs packages, as before.

[jdeflander]

I previously added the $ prompt to distinguish between terminal input and output (mainly useful in the usage example), so I would either drop it or keep it everywhere for consistency.

[BoKleynen]

I removed it here to make copy-pasting easier (yeah i'm that lazy), but for consistency it's probably better to readd the $ everywhere then because otherwise you won't be able to make that distinction in the usage section.