Add architecture information and images (open-telemetry#179)

* Add architecture information and images

* Address feedback

docs/architecture.md

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+# Architecture
+
+This distribution can be deployed in the below ways.
+
+## Helm or Installer Script (Recommended)
+
+![splunk-otel-collector-recommended-arch](images/splunk-otel-collector-recommended-arch.png)
+
+If Helm or the installer script is used then the architecture will look similar
+to above. The default configuration of the components is as follows:
+
+- [splunk-otel-collector (otelcol)](https://github.com/signalfx/splunk-otel-collector/blob/main/cmd/otelcol/config/collector/agent_config.yaml)
+- [td-agent (fluentd)](https://github.com/signalfx/splunk-otel-collector/tree/main/internal/buildscripts/packaging/fpm/etc/otel/collector/fluentd)
+
+## Standalone
+
+![splunk-otel-collector-standalone-arch](images/splunk-otel-collector-standalone-arch.png)
+
+If deployed in standalone, only the splunk-otel-collector is deployed and
+configured. Note the default configuration for standalone is gateway-mode instead of agent-mode like above:
+
+- [splunk-otel-collector (otelcol)](https://github.com/signalfx/splunk-otel-collector/blob/main/cmd/otelcol/config/collector/gateway_config.yaml)
+
+## Advanced
+
+![splunk-otel-collector-recommended-gateway-arch](images/splunk-otel-collector-recommended-gateway-arch.png)
+
+It is possible to manually configure the splunk-otel-collector running as an
+agent to send to a splunk-otel-collector gateway instance/cluster. This
+requires changing the pipeline exporters in the agent to point to the gateway.