Update README.md

Checking that GO111MODULE="auto" helps prevent the error that package 'github.com/dgraph-io/badger/v2' cannot be found.

README.md

@@ -59,6 +59,8 @@ $GOPATH/bin/sloop
 
 When complete, you should have a running Sloop version accessing the current context from your kubeConfig. Just point your browser at http://localhost:8080/
 
+**NOTE**: Before running ```make```, run ```go env``` and check that the module environment variable is set to ```GO111MODULE=auto```. If it is not run ```go env -w GO111MODULE=auto``` and then ```make```.
+
 Other makefile targets:
 
 * *docker*: Builds a Docker image.