Add Install section to README.md

I want quick access to install information. without need to click around.
put an install guide (usually one line go install.... into the README.

there you go I just found it by GOOGLING install go-migrate which pointed to the README.md in cmd/migrate.

which is what I thought: just one line: $ go install -tags 'postgres' github.com/golang-migrate/migrate/v4/cmd/migrate@latest

imagine, I had to navigate away from your github repo and google it just to come back to the same repo but a different directory.

Mention CLI install instructions in main README

Addressed in fccd197

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