Docs - improve information around removing default authenticators

[antons-]

I've just started playing around with this module.

The docs here https://github.com/silverstripe/silverstripe-ldap/blob/master/docs/en/developer.md#bypass-auto-login allude that you can remove the default member authenticator, but it doesn't actually tell you how.

In my case, I always wanted to force LDAP authentication.

I found the only way to do this was to first remove the default authenticator, then load the LDAP config. Very similar to the config here: https://github.com/bigfork/silverstripe-oauth-login#replacing-the-default-authenticator. I was only aware of this code as I had recently played around with that other module.

This should be included in the docs to make it easy for others - if you agree I'll create a PR.

[robbieaverill]

We'd welcome any improvements to the documentation. Thanks!