README.md: fix install instructions when used with modules

[kakulukia]

What version of protobuf and what language are you using?
Version: none yet

What did you do?
I followed the installation instructions telling me to go get -u github.com/golang/protobuf/protoc-gen-go

What did you expect to see?
Something.

What did you see instead?
go: cannot find main module; see 'go help modules'

Anything else we should know about your project / environment?
Ubuntu with a fresh installation of go

[neild]

This command doesn't work with module support turned on in the go command. The following will work for now:

GO111MODULE=off go get -u github.com/golang/protobuf/protoc-gen-go

We need to update the documentation with something that will continue to work after module support is enabled by default.

[kakulukia]

okay, thx

[neild]

Reopening, because this really is a problem with the documentation. (And thanks for reporting it!)

[dsnet]

This command doesn't work with module support turned on in the go command.

I'm not sure how to reproduce this failure. On go1.11, I run go get -u github.com/golang/protobuf/protoc-gen-go and it works regardless of whether I invoke it inside or outside the repo root.

[neild]

Set GO111MODULE=on and it should fail.

[dsnet]

This issue is blocked on golang/go#24250 since there are limitations to the Go toolchain for what can be done here.

[dsnet]

On Go1.16, you can install a binary from outside a module with:

go install google.golang.org/protobuf/cmd/protoc-gen-go@latest