Update README to address Simplecov intermittently dropping coverage

[darronschall]

After first integrating flatware, I've seen 2% drops in coverage in every 1-in-3 full suite runs, with a different worker missing from the final Coverage report generated for ... listing each time.

From what I can tell, flatware rspec returns to its top-level exit the moment the DRb sink has seen every worker report its final result — but a worker's at_exit (where SimpleCov stores its resultset slice to disk) runs after the last DRb report goes over the wire. So the parent's own SimpleCov at_exit (which merges all workers' slices into the final report) can fire before the slowest worker finishes writing.

The solution appears to be adding a Process.waitall barrier inside an at_exit registered from before_fork (after require 'rails_helper' has loaded SimpleCov). LIFO at_exit ordering means the barrier fires before SimpleCov's at_exit, so the merge sees the full set of workers' resultsets. The handler is gated to Process.pid == parent_pid; it no-ops in worker forks.

I'm running this setup in production now and have not seen any coverage drops.

Also, while I had the SimpleCov section open, I added a note for the SimpleCov.minimum_coverage_by_file 0 companion call in the after_fork example, since SimpleCov's default at_fork lambda clears the aggregate minimum_coverage gate but not the per-file one (so workers using the per-file gate kill themselves before their slice can merge).