README: update instructions to install CLI

[MarkLodato]

• Fix the go get command, explaining that it now only works for installing libraries, and add the now-mandatory @<version> suffix.
• Add instructions for using go install to run the binary.

[nigeltao]

@dmitshur two questions.

1. Can you remind me whether github.com/golang/foo PRs need a CLA?
2. Is the @latest in go get github.com/golang/snappy@latest an idiomatic recommendation?

[dmitshur]

@nigeltao Yes, I expect a passing CLA check is required (though I'm less familiar with snappy-specific details compared to the more common golang.org/x repos). This PR has it passing from looking at the Checks tab. We should ideally configure this repo's branch protection to require that check, so it's more clear.

For 2, I think it is better to leave out @latest from the go get command, and keep it as just go get github.com/golang/snappy. In supported versions of Go that defaults to @upgrade, which is very similar to @latest but behaves slightly better in rare edge cases. (It is good and necessary to include @latest in the go install command.)
So my suggestion is to keep go get github.com/golang/snappy and add go install github.com/golang/snappy/cmd/snappytool@latest.

[MarkLodato]

Done. PTAL. Thanks for the prompt review!