Add documentation guidelines and improve Jaeger exporter readme (#2082)

* Add documentation guidelines and improve Jaeger exporter readme

* Improve docs

* Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -113,6 +113,7 @@ A PR is considered to be **ready to merge** when:
   one day and may be merged with a single Maintainer's approval.
 * `CHANGELOG.md` has been updated to reflect what has been
   added, changed, removed, or fixed.
+* `README.md` has been updated if necessary.
 * Urgent fix can take exception as long as it has been actively
   communicated.
 
@@ -143,6 +144,26 @@ patterns in the spec.
 For a deeper discussion, see
 [this](https://github.com/open-telemetry/opentelemetry-specification/issues/165).
 
+## Documentation
+
+Each non-example Go Module should have its own `README.md` containing:
+
+- A pkg.go.dev badge which can be generated [here](https://pkg.go.dev/badge/).
+- Brief description.
+- Installation instructions (and requirements if applicable).
+- Hyperlink to an example. Depending on the component the example can be:
+  - An `example_test.go` like [here](exporters/stdout/stdouttrace/example_test.go).
+  - A sample Go application with its own `README.md`, like [here](example/zipkin).
+- Additional documentation sections such us:
+  - Configuration,
+  - Contributing,
+  - References.
+
+[Here](exporters/jaeger/README.md) is an example of a concise `README.md`.
+
+Moreover, it should be possible to navigate to any `README.md` from the
+root `README.md`.
+
 ## Style Guide
 
 One of the primary goals of this project is that it is actually used by

exporters/jaeger/README.md

@@ -1,13 +1,50 @@
 # OpenTelemetry-Go Jaeger Exporter
 
-OpenTelemetry Jaeger exporter
+[![Go Reference](https://pkg.go.dev/badge/go.opentelemetry.io/otel/exporters/jaeger.svg)](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger)
+
+[OpenTelemetry span exporter for Jaeger](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk_exporters/jaeger.md) implementation.
 
 ## Installation
 
 ```
 go get -u go.opentelemetry.io/otel/exporters/jaeger
 ```
 
-## Maintenance
+## Example
+
+See [../../example/jaeger](../../example/jaeger).
+
+## Configuration
+
+The exporter can be used to send spans to:
+
+- Jaeger agent using `jaeger.thrift` over binary thrift protocol via
+  [`WithAgentEndpoint`](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger#WithAgentEndpoint) option.
+- Jaeger collector using `jaeger.thrift` over HTTP via
+  [`WithCollectorEndpoint`](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger#WithCollectorEndpoint) option.
+
+### Environment Variables
+
+The following environment variables can be used
+(instead of options objects) to override the default configuration.
+
+| Environment variable              | Option                                                                                        | Default value                       |
+| --------------------------------- | --------------------------------------------------------------------------------------------- | ----------------------------------- |
+| `OTEL_EXPORTER_JAEGER_AGENT_HOST` | [`WithAgentHost`](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger#WithAgentHost) | `localhost`                         |
+| `OTEL_EXPORTER_JAEGER_AGENT_PORT` | [`WithAgentPort`](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger#WithAgentPort) |                                     |
+| `OTEL_EXPORTER_JAEGER_ENDPOINT`   | [`WithEndpoint`](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger#WithEndpoint)   | `http://localhost:14268/api/traces` |
+| `OTEL_EXPORTER_JAEGER_USER`       | [`WithUsername`](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger#WithUsername)   |                                     |
+| `OTEL_EXPORTER_JAEGER_PASSWORD`   | [`WithPassword`](https://pkg.go.dev/go.opentelemetry.io/otel/exporters/jaeger#WithPassword)   |                                     |
+
+Configuration using options have precedence over the environment variables.
+
+## Contributing
+
+This exporter uses a vendored copy of the Apache Thrift library (v0.14.1) at a custom import path.
+When re-generating Thrift code in the future, please adapt import paths as necessary.
+
+## References
 
-This exporter uses a vendored copy of the Apache Thrift library (v0.14.1) at a custom import path. When re-generating Thrift code in future, please adapt import paths as necessary.
+- [Jaeger](https://www.jaegertracing.io/)
+- [OpenTelemetry to Jaeger Transformation](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk_exporters/jaeger.md)
+- [OpenTelemetry Environment Variable Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md)