Add doc for plugin manifest explaining devfs_ruleset

(cherry picked from commit 49e6280)

Author: sonicaj

doc/source/plugins.rst

@@ -79,3 +79,48 @@ installed to a FreeNAS system, then the note of the plugin is changed:
 
 The process for upgrading and updating plugins is exactly the same as
 normal jails. See :ref:`Updating Jails` or :ref:`Upgrading Jails` .
+
+
+**Plugin Manifest Example**
+
+
+Following is an example of a plugin manifest:
+
+.. sourcecode:: json
+
+    {
+        "name": "default_jail_name_here",
+        "release": "11.3-RELEASE",
+        "artifact": "https://github.com/git_path_to_plugin_repo",
+        "official": false,
+        "properties": {
+            "nat": 1,
+            "nat_forwards": "tcp(7878:7878)"
+        },
+        "devfs_ruleset": {
+            "paths": {"bpf*": null},
+            "includes": []
+        },
+        "pkgs": [
+        ],
+        "packagesite": "http://pkg.FreeBSD.org/${ABI}/latest",
+        "fingerprints": {
+            "iocage-plugins": [
+                {
+                    "function": "sha256",
+                    "fingerprint": "b0170035af3acc5f3f3ae1859dc717101b4e6c1d0a794ad554928ca0cbb2f438"
+                }
+            ]
+        },
+        "revision": "0"
+    }
+
+* **devfs_ruleset**: It should be a valid dictionary object where "paths" must be specified. Value of "paths" is a
+  dictionary where keys are the path to be added and the value is the mode to be used. `null` translates to `unhide`.
+  For any include specified, please refer to the following example which shows how each path specified is added as
+  iocage dynamically generates a devfs ruleset and how an include is added to the ruleset:
+
+.. sourcecode:: shell
+
+    devfs rule -s ruleset_number add path bpf* unhide
+    devfs rule -s ruleset_number add include include_value