[examples] Overhaul the example service README.

The documentation for the example services has fallen out of date, as
reported in #466. This patch updates the README to include the latest
usage instructions.

examples/example_compiler_gym_service/README.md

@@ -9,46 +9,60 @@ RPC interface that the frontend interacts with.
 
 This directory contains an example backend service implemented in C++ and
 Python. Both implementations have the same features. They don't do any actual
-compilation, but can be used as a starting point for writing new services, or
-for debugging and testing frontend code.
+compilation, but can be used as a starting point for adding support for new
+compilers, or for debugging and testing frontend code.
 
-Features:
+If you have any questions please [file an
+issue](https://github.com/facebookresearch/CompilerGym/issues/new/choose).
 
-* Enforces the service contract, e.g. `StartSession()` must be called before
-  `EndSession()`, list indices must be in-bounds, etc.
-* Implements all of the RPC endpoints.
+
+## Features
+
+* A static action space with three items: `["a", "b", "c"]`. The action space
+  never changes. Actions never end the episode.
+* There are two observation spaces:
+  * `ir` which returns the string "Hello, world!".
+  * `features` which returns an `int64_list` of `[0, 0, 0]`.
+* A single reward space `runtime` which returns 0.
 * It has a single dataset "benchmark://example-v0" with two programs "foo" and
   "bar".
-* It has a static action space with three items: `["a", "b", "c"]`. The action
-  space never changes. Actions never end the episode.
-* There are two observation spaces:
-    * `ir` which returns the string "Hello, world!".
-    * `features` which returns an `int64_list` of `[0, 0, 0]`.
-* There is a single reward space `runtime` which returns 0.
-* Supports default observation and reward spaces.
 
-See [service_cc/ExampleService.h](service_cc/ExampleService.h) for the C++
-service implementation,
-[service_py/example_service.py](service_py/example_service.py) for the Python
-version, [__init__.py](__init__.py) for a python module that registers this
-service with the gym on import, and [env_tests.py](env_tests.py) for tests.
 
+## Implementation
+
+There are two identical service implementations, one in Python, one in C++. See
+[service_cc/ExampleService.h](service_cc/ExampleService.h) for the C++ service,
+and [service_py/example_service.py](service_py/example_service.py) for the
+Python service. The module [__init__.py](__init__.py) defines the reward space,
+dataset, and registers two new environments using these services.
+
+The file [demo.py](demo.py) demonstrates how to use these example environments
+using CompilerGym's bazel build system. The file [env_tests.py](env_tests.py)
+contains unit tests for the example services. Because the Python and C++
+services implement the same interface, the same tests are run against both
+environments.
 
 ## Usage
 
-Start an example service using:
+Run the demo script using:
 
 ```sh
-$ bazel run -c opt //examples/example_compiler_gym_service/service -- \
-      --port=8080 --working_dir=/tmp
+$ bazel run -c opt //examples/example_compiler_gym_service:demo
 ```
 
-The service never terminates and does not print logging messages. Interact with
-the RPC endpoints using your frontend of choice, for example, the manual
-environment:
+Run the unit tests using:
 
 ```sh
-$ bazel run -c opt //compiler_gym/bin:manual_env -- --service=localhost:8080
+$ bazel test //examples/example_compiler_gym_service/...
 ```
 
-Kill the service using C-c when you are done.
+### Using the python service without bazel
+
+Because the python service contains no compiled code, it can be run directly as
+a standalone script without using the bazel build system. From the root of the
+CompilerGym repository, run:
+
+```sh
+$ cd examples
+$ python3 example_compiler_gym_service/demo_without_bazel.py
+```