Add a README to the exporter helper (#1922)

This can be referenced by exporter leveraging this helper instead of
duplicating the configuration parameters.

exporter/exporterhelper/README.md

@@ -0,0 +1,25 @@
+# Exporter Helper
+
+This is a helper exporter that other exporters can depend on. Today, it
+primarily offers queued retries capabilities.
+
+> :warning: This exporter should not be added to a service pipeline.
+
+## Configuration
+
+The following configuration options can be modified:
+
+- `retry_on_failure`
+  - `enabled` (default = true)
+  - `initial_interval` (default = 5s): Time to wait after the first failure before retrying; ignored if `enabled` is `false`
+  - `max_interval` (default = 30s): Is the upper bound on backoff; ignored if `enabled` is `false`
+  - `max_elapsed_time` (default = 120s): Is the maximum amount of time spent trying to send a batch; ignored if `enabled` is `false`
+- `sending_queue`
+  - `enabled` (default = true)
+  - `num_consumers` (default = 10): Number of consumers that dequeue batches; ignored if `enabled` is `false`
+  - `queue_size` (default = 5000): Maximum number of batches kept in memory before data; ignored if `enabled` is `false`;
+  User should calculate this as `num_seconds * requests_per_second` where:
+    - `num_seconds` is the number of seconds to buffer in case of a backend outage
+    - `requests_per_second` is the average number of requests per seconds.
+
+The full list of settings exposed for this helper exporter are documented [here](factory.go).