[docs] update README to include git install instruction

[Xirui]

Using go get -u github.com/cweill/gotests/... to install would give following error message.

go get: installing executables with 'go get' in module mode is deprecated.
	Use 'go install pkg@version' instead.
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

Instruction to use go install is added to README for Go version >= 1.17.

[Xirui]

@sleroq No worries. It still needs 1 approving review?

[sleroq]

@sleroq No worries. It still needs 1 approving review?

Yes, I think, we need a reviewer with write access