Add DOUGAL_ROOT symlink to node_modules.

This can be used as a shortcut when requiring a module from deep
within the file hierarchy, e.g., instead of:

require("../../../../lib/db");

one can do:

require("DOUGAL_ROOT/lib/db");

lib/www/server/package.json

@@ -5,7 +5,7 @@
   "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
-    "postinstall": "redoc-cli bundle ./spec/openapi.yaml -o ./spec/openapi.html"
+    "postinstall": "[[ -e ./node_modules/DOUGAL_ROOT ]] || ln -s .. ./node_modules/DOUGAL_ROOT; redoc-cli bundle ./spec/openapi.yaml -o ./spec/openapi.html"
   },
   "author": "Aaltronav s.r.o.",
   "license": "UNLICENSED",