[bug] README #692

Describe the bug

A clear and concise description of what the bug is.
amw := authenticationMiddleware{}
when use Populate panic
below is right
amw := authenticationMiddleware{tokenUsers: make(map[string]string)}
…

Versions

Go version: go version
package version: run git rev-parse HEAD inside the repo

…

Steps to Reproduce

How can the bug be triggered?

…

Expected behavior

What output or behaviour were you expecting instead?

…

Code Snippets

A minimum viable code snippet can be useful! (use backticks to format it).

…