Add gpu_usage and model_usage in YAML config docs

Closes #181 (closed)

docs/contents/workers/yaml.md

@@ -67,6 +67,21 @@ include:
     - `word-segmenter`
     - `paragraph-creator`
 
+`gpu_usage`
+: Whether or not this worker requires or supports GPUs. Defaults to `disabled`. May take one of the following values:
+
+    `required`
+    : This worker requires a GPU, and will only be run on Ponos agents whose hosts have a GPU.
+
+    `supported`
+    : This worker supports using a GPU, but may run on any available host, including those without GPUs.
+
+    `disabled`
+    : This worker does not support GPUs. It may run on a host that has a GPU, but it will ignore it.
+
+`model_usage`
+: Boolean. Whether or not this worker requires a model version to run. Defaults to `false`.
+
 `docker`
 : Regroups Docker-related configuration attributes:
     <!--