[receiver/lightprometheus] Add README.md (#3049)

Co-authored-by: Ryan Fitzpatrick <10867373+rmfitzpatrick@users.noreply.github.com>

internal/receiver/lightprometheusreceiver/README.md

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+# Light Prometheus Receiver
+
+| Status                   |               |
+| ------------------------ |---------------|
+| Stability                | [development] |
+| Supported pipeline types | metrics       |
+| Distributions            | splunk        |
+
+[development]: https://github.com/open-telemetry/opentelemetry-collector#development
+
+## Overview
+
+Light Prometheus Receiver is a component that can scrape Prometheus metrics from a Prometheus exporter endpoint and
+convert them to OTLP metrics. It is intended to be used as a replacement for the [Simple Prometheus 
+Receiver](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/receiver/simpleprometheusreceiver)
+as it is more efficient and has a smaller memory footprint.
+
+The receiver is under active development which means that configuration interface can change.
+
+## Configuration
+
+The following settings are required:
+
+- `endpoint` (no default): Address to request Prometheus metrics. This is the same endpoint that 
+  Prometheus scrapes to collect metrics. IMPORTANT: This receiver currently does require the metric path to be included
+  in the endpoint. For example, if the endpoint is `localhost:1234`, the metrics path must be included, e.g.
+  `localhost:1234/metrics`. This likely will be changed in the future.
+
+The following settings can be optionally configured:
+
+- `collection_interval` (default = 30s): The internal at which metrics should be scraped by this receiver.
+- `resource_attributes`: Resource attributes to be added to all metrics emitted by this receiver. The following options
+  are available to configure resource attributes:
+  - `service.name`:
+    - `enabled`: (default: true)
+  - `service.instance.id`:
+    - `enabled`: (default: true)
+  - `net.host.name`:
+    - `enabled`: (default: false)
+  - `net.host.port`:
+    - `enabled`: (default: false)
+  - `http.scheme`:
+    - `enabled`: (default: false)
+- [HTTP Client Configuration options](https://github.com/open-telemetry/opentelemetry-collector/tree/main/config/confighttp#client-configuration)
+
+The full list of configuration options exposed for this receiver are documented [here](./config.go) with an example [here](./testdata/config.yaml).