readme: mention Go modules

Author: vmihailenco

README.md

@@ -27,16 +27,16 @@ Examples: https://godoc.org/github.com/go-redis/redis#pkg-examples.
 
 ## Installation
 
-Install:
+go-redis requires a Go version with [Modules](https://github.com/golang/go/wiki/Modules) support and uses import versioning. So please make sure to initialize a Go module before installing go-redis:
 
 ```shell
-go get -u github.com/go-redis/redis
+go get -u github.com/go-redis/redis/v7
 ```
 
 Import:
 
 ```go
-import "github.com/go-redis/redis"
+import "github.com/go-redis/redis/v7"
 ```
 
 ## Quickstart