Update EnvVars.md (#78)

Author: Humbertzhang

docs/EnvVars.md

@@ -5,7 +5,7 @@ Environment Variable | Description | Default
 | `SW_AGENT_NAME` | The name of the Python service | `Python Service Name` |
 | `SW_AGENT_INSTANCE` | The name of the Python service instance | Randomly generated |
 | `SW_AGENT_COLLECTOR_BACKEND_SERVICES` | The backend OAP server address | `127.0.0.1:11800` |
-| `SW_AGENT_PROTOCOL` | The protocol to communicate with the backend OAP, `http` or `grpc`, **we highly suggest using `grpc` in production as it's well optimized than `http`** | `grpc` |
+| `SW_AGENT_PROTOCOL` | The protocol to communicate with the backend OAP, `http`, `grpc` or `kafka`, **we highly suggest using `grpc` in production as it's well optimized than `http`**. The `kafka` protocol provides an alternative way to submit data to the backend. | `grpc` |
 | `SW_AGENT_AUTHENTICATION` | The authentication token to verify that the agent is trusted by the backend OAP, as for how to configure the backend, refer to [the yaml](https://github.com/apache/skywalking/blob/4f0f39ffccdc9b41049903cc540b8904f7c9728e/oap-server/server-bootstrap/src/main/resources/application.yml#L155-L158). | unset |
 | `SW_AGENT_LOGGING_LEVEL` | The logging level, could be one of `CRITICAL`, `FATAL`, `ERROR`, `WARN`(`WARNING`), `INFO`, `DEBUG` | `INFO` |
 | `SW_AGENT_DISABLE_PLUGINS` | The name patterns in CSV pattern, plugins whose name matches one of the pattern won't be installed | `''` |