Add a page on contributing benchmarks

This is taken from godot-benchmarks' `CONTRIBUTING.md` with some
information updated and an introductory paragraph added.

other/benchmarks.rst

@@ -3,4 +3,101 @@
 Contributing benchmarks
 =======================
 
-TODO
+To keep track of Godot performance across commits,
+we have a `benchmarks repository <https://github.com/godotengine/godot-benchmarks>`__
+that is run on a dedicated server every day. You can look at the results on
+`benchmarks.godotengine.org <https://benchmarks.godotengine.org>`__.
+
+While we have a set of benchmarks that we run on a daily basis, we are always
+looking for new benchmarks to add to the repository. If you have an idea for a
+new benchmark, please submit a pull request for it on the
+`Godot benchmarks <https://github.com/godotengine/godot-benchmarks>`__ repository.
+
+This page describes the process of adding a new benchmark to the repository.
+It's recommended to open one pull request per set of benchmarks. If you create
+multiple benchmarks in unrelated categories, please open separate pull requests
+for each.
+
+Adding new benchmarks
+---------------------
+
+You can add new benchmarks by following these steps:
+
+Create new benchmark
+^^^^^^^^^^^^^^^^^^^^
+
+Create a new script with ``snake_case`` naming that
+``extends Benchmark``, then save it in one of the existing folders
+depending on its category. If your benchmark does not suit any existing
+category or subcategory, you can also create a new folder in the
+repository's root folder.
+
+Configure the benchmark
+^^^^^^^^^^^^^^^^^^^^^^^
+
+In the ``_init()`` function of your script, configure your benchmark by
+initializing any desired ``Benchmark`` variables to true:
+
+- ``test_render_cpu``: Enable this for rendering benchmarks. Leave it
+  disabled for other benchmarks.
+- ``test_render_gpu``: Enable this for rendering benchmarks. Leave it
+  disabled for other benchmarks.
+- ``test_idle``: Enable this for non-rendering CPU-intensive
+  benchmarks. Leave it disabled for other benchmarks.
+- ``test_physics``: Enable this for physics benchmarks. Leave it
+  disabled for other benchmarks.
+- Leaving all of these disabled will only time the initial call to your
+  ``benchmark_`` function (see below).
+
+Implement the benchmark
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The runtime will scan your script for functions beginning with
+``benchmark_``. For basic benchmarks, writing your code inside such a
+function is sufficient for it to be automatically registered and become
+available for benchmarking by the user. The runtime will end the
+benchmark once your function returns and report the amount of time spent
+running inside it.
+
+For more advanced benchmarks, the runtime expects the function to return
+a ``Node``. Return the appropriate node for your benchmark to
+signal to the runtime that it should continue measuring system resource
+usage even after your function returns. The node type you should return
+depends on what kind of benchmark it is:
+
+- For 3D rendering benchmarks, the root node should be Node3D.
+- For 2D rendering benchmarks, the root node should be Node2D.
+- For UI benchmarks, the root node should be Control.
+- For scripting or miscellaneous benchmarks, the root node should be Node.
+
+The runtime will add your node to the scene as a root node, leave it running
+for a fixed amount of time (5 seconds), and then end the benchmark and
+report the various metrics configured above.
+
+Remember to follow the
+`GDScript style guide <https://docs.godotengine.org/en/latest/tutorials/scripting/gdscript/gdscript_styleguide.html>`__
+when writing new scripts. Adding type hints is recommended whenever
+possible, unless you are specifically benchmarking non-typed scripts.
+
+.. note::
+
+   C# benchmark functions must begin with ``Benchmark``, instead of
+   ``benchmark_``. For C# benchmarks to be available, you must use the
+   .NET version of the engine.
+
+Test the benchmark
+^^^^^^^^^^^^^^^^^^
+
+1. Run the project in the editor.
+2. Check the checkbox next to your newly added benchmark and click
+   **Run** in the bottom-right corner.
+3. Wait for the benchmark to complete.
+4. If all goes well, the benchmark completion time should appear in the
+   table for all enabled benchmarks (and all relevant metrics). Metrics
+   that were disabled on a specific benchmarks will not display any
+   result.
+
+If the benchmark works as expected, congratulations! You can now open a
+pull request for it on the
+`Godot benchmarks <https://github.com/godotengine/godot-benchmarks>`__
+repository.