Add user management to set-up workflow

1. Add how-to for user management to README (Firebase 2.0 only)
2. Add error message constant to constants.js to display specific error to user
  if auth is not enabled in Firebase

README.md

@@ -81,7 +81,7 @@ The test suite is run using `npm test` and `npm run test:watch`. Individual unit
         // auth can't be null to make/edit post
         // if the post exists, auth.uid must match creatorUID
         ".write": "(auth != null && !data.exists()) || data.child('creatorUID').val() === auth.uid",
-          
+
         // make sure all 5 fields are present before saving a new post
         // leave 'isDeleted' when deleting a post
         ".validate": "newData.hasChildren(['title', 'url', 'creator', 'creatorUID', 'time']) ||
@@ -124,12 +124,12 @@ The test suite is run using `npm test` and `npm run test:watch`. Individual unit
     "comments": {
       ".read": true,
       ".indexOn": ["postId","creatorUID","time"],
-      
+
       "$comment_id": {
         ".write": "auth != null && (!data.exists() || data.child('creatorUID').val() === auth.uid)",
         ".validate": "newData.hasChildren(['postId', 'text', 'creator', 'creatorUID', 'time']) &&
                       (newData.child('text').isString() && newData.child('text').val() != '')",
-        
+
         "upvotes": {
           // upvotes must be writable by anyone logged in
           ".write": "auth != null",
@@ -169,4 +169,12 @@ The test suite is run using `npm test` and `npm run test:watch`. Individual unit
 
   }
 }
-```
+```
+
+## Firebase Authentication
+
+To set up users, from your Firebase dashboard:
+
+1. Click Auth
+1. Click users
+1. Enable Email/password sign-in method

src/js/util/constants.js

@@ -10,6 +10,7 @@ export const errorMessages = {
     NO_USERNAME: 'You have to enter a username.',
     EMAIL_TAKEN: 'That email is taken.',
     USERNAME_TAKEN: 'That username is taken.',
+    AUTHENTICATION_DISABLED: 'Your administrator needs to configure Firebase authentication',
     generic: 'Something went wrong.'
 };