Document decision to not send cache headers by default in development

[evanob]

I noticed when using Phoenix.Sync locally that the browser wasn't caching requests as I hoped. After digging into the code I discovered that I need to configure send_cache_headers?: true. This worked nicely, but it would be helpful to document this (along with trade-offs and why I might not want to do this).

phoenix_sync/lib/phoenix/sync/electric.ex

Lines 386 to 394 in 405710e

	defp env_defaults(opts, :dev) do
	# can't use new tmp dir for every run in dev because the storage
	# path must remain consistent between invocations of children(), plug_opts()
	# and client()...
	# if we want to use emphemeral dir for dev storage then we have to persist
	# the storage_dir into the application config.
	opts
	|> Keyword.put_new(:send_cache_headers?, false)
	end