Document go1.13+ requirement

Hi,

I am trying to install snowflake standalone. But I have an issue with module crypto/ed25519`

Steps to reproduce

git clone https://git.torproject.org/pluggable-transports/snowflake.git
cd snowflake
cd proxy
go get 

Result

build git.torproject.org/pluggable-transports/snowflake.git/proxy: cannot find module for path crypto/ed25519

Setup

• Debian 10
• go version go1.11.6 linux/amd64