Add readme about how to provide configuration to the collector

This explains only how to provide the configuration via the `--config` file, after #4742 will update the embedded values option.

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

service/README.md

@@ -0,0 +1,36 @@
+# OpenTelemetry Collector Service
+
+## How to provide configuration
+
+The `--config` flag accepts either a file path or values in the form of a config URI `"<scheme>:<opaque_data>"`. Currently, the
+OpenTelemetry Collector supports the following providers `scheme`:
+- [file](../confmap/provider/fileprovider/provider.go) - Reads configuration from a file. E.g. `file:path/to/config.yaml`.
+- [env](../confmap/provider/envprovider/provider.go) - Reads configuration from an environment variable. E.g. `env:MY_CONFIG_IN_AN_ENVVAR`.
+- [yaml](../confmap/provider/yamlprovider/provider.go) - Reads configuration from yaml bytes. E.g. `yaml:exporters::logging::loglevel: debug`.
+
+For more technical details about how configuration is resolved you can read the [configuration resolving design](../confmap/README.md#configuration-resolving).
+
+### Single Config Source
+
+1. Simple local file:
+
+    `./otelcorecol --config=examples/local/otel-config.yaml`
+
+2. Simple local file using the new URI format:
+
+    `./otelcorecol --config=file:examples/local/otel-config.yaml`
+
+3. Config provided via an environment variable:
+
+    `./otelcorecol --config=env:MY_CONFIG_IN_AN_ENVVAR`
+
+
+### Multiple Config Sources
+
+1. Merge a `otel-config.yaml` file with the content of an environment variable `MY_OTHER_CONFIG` and use the merged result as the config:
+
+    `./otelcorecol --config=file:examples/local/otel-config.yaml --config=env:MY_OTHER_CONFIG`
+
+2. Merge a `config.yaml` file with the content of a yaml bytes configuration (overwrites the `exporters::logging::loglevel` config) and use the content as the config:
+
+    `./otelcorecol --config=file:examples/local/otel-config.yaml --config="yaml:exporters::logging::loglevel: info"`