docs(readme): the README says what is true now, without the old notes

Behavior changes held one note per past fix; it keeps what an update
changes for operators and users. Migration is a section of bullets.
Docker cannot keep options that exist only in settings.py (LDAP,
OTP_ENABLED, LOGIN_TRUSTED_PROXIES): the README shows how to mount
one. The OpenLDAP examples use OpenLDAP names, the variables table
has ADMIN_USERNAME and ADMIN_PASSWORD, Contents lists every section,
and features name backups and the languages.

Removed: screenshots of the 2020 interface, the Gitpod badge and its
TODO-only config, and doc/readme.md, which held one word.

doc/images/readme.md

@@ -1 +0,0 @@
-IMAGES

doc/readme.md

@@ -1 +0,0 @@
-README