Document the load balancer model in the README

[sudomateo]

Context

README.adoc covers none of the load balancer behavior users will actually interact with. Every item below will surprise a user today.

Scope

Add a load balancer section to the README documenting:

• The three service annotations: oxide.computer/floating-ip, oxide.computer/floating-ip-pool, oxide.computer/floating-ip-version (including mutual-exclusion rules).
• That the "load balancer" is a floating IP pinned to a single node chosen alphabetically — a bandwidth bottleneck, with failover happening on reconcile cadence rather than instantly.
• That externalTrafficPolicy: Local is rejected (and why).
• Which Service fields are unsupported (loadBalancerSourceRanges, loadBalancerIP, dual-stack ipFamilies).

Done when

A new user can predict LB behavior from the README alone.