(doc) update README.md and client package.json

- specify nodejs version
- clearer setup process
- cleanup of phoenix doc artifacts
- prioritize manual install

js/package.json

@@ -2,6 +2,9 @@
   "name": "mobilizon",
   "version": "0.1.0",
   "private": true,
+  "engines" : {
+    "node" : ">=11.4.0" 
+  },
   "scripts": {
     "dev": "vue-cli-service serve",
     "build": "vue-cli-service build --modern",