docs: add escape example for cache_from parameter (#148)

_docs/data/data.yaml

@@ -147,7 +147,20 @@ properties:
     required: false
 
   cache_from:
-    description: Images to consider as cache sources.
+    description: |
+      Images to consider as [cache sources](https://docs.docker.com/engine/reference/commandline/buildx_build/#cache-from). To properly work,
+      commas used in the cache source entries need to be escaped:
+
+      ```Yaml
+      - name: Build
+        image: thegeeklab/drone-docker-buildx:20
+        settings:
+          repo: example/repo
+          cache_from:
+            # using quotes double-escaping is required
+            - "type=registry\\\\,ref=example"
+            - 'type=foo\\,ref=bar'
+      ```
     type: list
     required: false