Add lines to readme.md

Hi,

I would like to suggest adding the following lines to the readme.md under the code generation plugin paragraph:

"Make sure they are both executable and are discoverable from your PATH. Also make sure to rename the executable to protoc-gen-grpc-web by removing any os/version info from the name of the executable."

I made a pull request for it, but it seems I need to sign a whole bunch of legal stuff, which honestly I don't want to do. I just want these lines to be added so it is clear to users that the default executable needs to be renamed, besides just being put on the path.

Kind regards.