add nedb demo

src/database/nedb/demo/demo.js

@@ -0,0 +1,42 @@
+var NeDB = require('nedb')
+
+var db = new NeDB({
+    filename: './user.db',
+    autoload: true,
+})
+
+db.insert({
+    name: 'Alice',
+    age: 20,
+    rank: 1,
+}, function(err, doc) {
+    console.log('inserted:', doc)
+})
+
+db.find({
+    name: 'Alice',
+}, function(err, docs) {
+    console.log('Alice found:', docs)
+})
+
+db.update({
+    name: 'Alice',
+}, {
+    $set: {
+        age: 21,
+    },
+}, function(err, n) {
+    console.log('docs updated:', n)
+})
+
+db.find({
+    name: 'Alice',
+}, function(err, docs) {
+    console.log('Alice changed:', docs)
+})
+
+db.remove({
+    name: 'Alice',
+}, function(err, n) {
+    console.log('docs deleted:', n)
+})