Play Framework

docs(readme): describe fork's Netty 4.2 / VT / no-legacy-1.x deviations

Adds an "About this fork" section between the upstream description and
the Getting Started instructions, calling out the three substantive
deviations from upstream Play 1:

  * Full Netty 3 → 4.2.x migration with refcount discipline, body spooling
    above play.netty.spoolThresholdBytes, and play.netty.maxContentLength
    body caps.
  * Unconditional virtual-thread execution for request invocation, jobs,
    and mail dispatch via VirtualThreadScheduledExecutor; Java 25+ required
    so JEP 491's no-pinning synchronized takes effect.
  * Removal of Continuations/Javaflow, the play.threads.virtual* /
    play.pool / play.jobs.pool toggles, ExecutorFacade, and deprecated
    executor mirrors. Boot-time WARN catches retired keys in application
    .conf (incl. profile-prefixed forms like %test.play.pool=3).

Upstream badges and documentation links are deliberately left as-is —
those describe Play upstream and are still useful for general framework
docs the fork didn't change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>