slog-configurator v1.2.1 — README only

The feature list now mentions what v1.2.0 added: caller-named environment
variables via Init(Options), and the logring in-memory ring. Both had their own
sections but were missing from the list a reader skims first.

Added a table of contents; the README had grown past the point where anything
below the fold is findable.

No library code changed.