Improve exporterhelper README

[araiu]

With the migration from batchprocessor to the exporterhelper we found there is a slight difference in the configuration.

Due to

opentelemetry-collector/exporter/exporterhelper/internal/queuebatch/config.go

Line 89 in 5675860

return errors.New("`min_size` must be less than or equal to `queue_size`")

starting the collector with

exporters:
  otlp:
    sending_queue:
      enabled: true
      batch:
        min_size: 8192

or

exporters:
  otlp:
    sending_queue:
      enabled: true
      queue_size: 1000
      batch:
        min_size: 8192

will fail to start.

Proposing to mention this condition in

opentelemetry-collector/exporter/exporterhelper/README.md

Line 39 in 5675860

- `min_size` (default = 8192): the minimum size of a batch;

[axw]

@araiu sounds sensible to me. Would you like to send a PR?

[araiu]

I'd be glad to . Thank you !