Add plugin isolation IPC scaffolding and host phase one docs (#5)

LanMountainDesktop.PluginTemplate/content/README.md

@@ -22,3 +22,4 @@ Update `plugin.json` fields as needed before release:
 - `description`
 - `author`
 - `version`
+- `runtime.mode` (`in-proc` by default, `isolated-background` for phase-1 worker mode)

LanMountainDesktop.PluginTemplate/content/plugin.json

@@ -6,5 +6,8 @@
   "version": "1.0.0",
   "apiVersion": "4.0.2",
   "entranceAssembly": "LanMountainDesktop.PluginTemplate.dll",
-  "sharedContracts": []
+  "sharedContracts": [],
+  "runtime": {
+    "mode": "in-proc"
+  }
 }