Summary

Document the discrete distribution APIs that are already available in bayes-rs. The README now shows Bernoulli, Binomial, and Poisson alongside the existing continuous distributions so users can find the count-valued distribution path without reading the source first.

Changes

• Lists Bernoulli, Binomial, and Poisson in the feature overview.
• Adds a discrete distributions README section covering DiscreteDistribution, pmf, log_pmf, and seeded sampling.
• Adds README example commands for discrete distributions, conjugate models, and serde-enabled diagnostics serialization.

Test plan

• Rust formatting check passed.
• Clippy passed with all targets and all features.
• Full test suite passed with all features.
• Documentation build passed with warnings denied.
• Benchmark compilation passed.
• README example commands and the new external seeded-sampling snippet were checked.