Update build binary step instruction on README

[gutakk]

Why

@Lahphim found the issue when building the binary (more information #46). But @junan point out that GO111MODULE will be ignored in Go 1.17 (more information #48 (comment))

Acceptance Criteria

Update the README to follow @junan suggestion

[malparty]

Not sure if related, but on my first attempt I had to "go mod init module name" before the command go build -o [...] worked.