Add changelog for 2.0.0b1

[consideRatio]

I've written up the Breaking changes and Deprecation headings manually, the rest is automated.

Before merge and release, I'd like to:

• have my open PRs considered
  • Add tls_kwargs config to configure underlying ldap3 package tls #273
  • Improve logging, docstring, and variable naming in resolve_username function #274
  • Add missing docs for search_filter and attributes and improve logging for search_filter #275
  • Require a unique DN to be found when using lookup_dn #276
• update any date on when we get the release out
• add the most recently merged PRs

I've requested review already though to focus on the Breaking changes and Deprecation headings as well as get a go-ahead for cutting 2.0.0b1.