Update installation instruction

[bokunodev]

The installation instruction is not working anymore, with latest go version.
From the readme :

$ cd $GOPATH/src/github.com/btcsuite/btcd
$ GO111MODULE=on go install -v . ./cmd/...

This should be changed, there is no $GOPATH/src with latest go. Module cache is placed in $GOPATH/pkg
I stumbled upon some ppl who struggle, trying to install this thing.

[bokunodev]

User forced to touch module cache because of this part replace github.com/btcsuite/btcd/btcutil => ./btcutil. what is the purpose anyway? (changing it wont fix the problem)
I'm aware, changing it might break existing build (it does), alternatively we should add git as dependency and ask user to clone the repo manually then build from there.

[bokunodev]

#1893

[chappjc]

Sorry to butt in again, but future release tags should really have a go.mod with no replace statements. Recent go versions even refuse to build such releases in the modern fashion:

$ go install -v github.com/btcsuite/[email protected]
go: github.com/btcsuite/[email protected] (in github.com/btcsuite/[email protected]):
	The go.mod file for the module providing named packages contains one or
	more replace directives. It must not contain directives that would cause
	it to be interpreted differently than if it were the main module.

However, building from a development branch to test something unreleased is different, and that certainly requires cloning the repo.

It all depends on what the README intends to describe. i.e. "How as an end-user can I build and install a release from source?" vs. "How can I build the latest development branch?"

[bokunodev]

we stuck on that. long term solution would be to separate each module to their own repo (out of scope of this PR).
should i change it to git clone -b <tag> [email protected]:btcsuite/btcd.git instead ?