Added export function from firestore

.gitignore

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+node_modules
+serviceAccountKey.json
+firestore-export.json

README.md

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 A script that help to export and import in Cloud Firestore
+
+# Requirements
+
+You need [NODE](https://nodejs.org/en/download/) or something that can run JAVASCRIPT (JS) file.
+
+Get **serviceAccount** JSON file from *Project Setting > SERVICE ACCOUNTS* in Firebase Console
+
+# Export database from Firestore
+
+This will help you create a backup of your collection from Firestore to a JSON file name **firestore-export.json** 
+
+```
+node export.js <your-collection-name>
+```
+
+# Import database to Firestore

export.js

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+var admin = require("firebase-admin");
+var fs = require('fs');
+var serviceAccount = require("./serviceAccountKey.json");
+
+var collectionName = process.argv[2];
+
+admin.initializeApp({
+  credential: admin.credential.cert(serviceAccount),
+  databaseURL: "https://ionic-firestore-dn.firebaseio.com"
+});
+
+var db = admin.firestore();
+
+var data = {};
+data[collectionName] = {};
+
+var results = db.collection(collectionName)
+.get()
+.then(snapshot => {
+  snapshot.forEach(doc => {
+    data[collectionName][doc.id] = doc.data();
+  })
+  return data;
+})
+.catch(error => {
+  console.log(error);
+})
+
+results.then(dt => {
+  // Write collection to JSON file
+  fs.writeFile("firestore-export.json", JSON.stringify(dt), function(err) {
+      if(err) {
+          return console.log(err);
+      }
+      console.log("The file was saved!");
+  });
+})