Bit of doc on data pack file systems.

src/main/resources/data/oc2/file_systems/README.md

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+It is possible to provide additional read-only data to virtual machines using data packs.
+
+For this to work, a descriptor file must be placed in the `file_systems` directory. The descriptor must be a JSON file
+with a name ending in `.fs.json`. The file must have the following format:
+
+```json
+{
+    "type": "<type>",
+    "<type specific data>": "<type specific data>"
+}
+```
+
+For example, to provide additional files for the layered 9pFS exposed to virtual machines, the type must be `virtio-9p`
+and the extra field `location` must be provided, containing a resource location pointing to the root of the path to
+expose. Example:
+
+```json
+{
+    "type": "virtio-9p",
+    "location": "oc2:file_systems/scripts"
+}
+```
+
+All files in paths exposed this way will be flagged non-executable by default. To mark a file executable, place
+a `.mcmeta` file with the following content next to:
+
+```json
+{
+    "attributes": {
+        "is_executable": true
+    }
+}
+```