Added changelog and updated readme for v3.1 (#432)

* added changelog for v3.1
* updated to v3.1 in readme

Author: wti806

README.md

@@ -63,17 +63,17 @@ Localized versions of the widget are available through the CDN. To use a localiz
 localized JS library instead of the default library:
 
 ```html
-<script src="https://www.gstatic.com/firebasejs/ui/3.0.0/firebase-ui-auth__{LANGUAGE_CODE}.js"></script>
-<link type="text/css" rel="stylesheet" href="https://www.gstatic.com/firebasejs/ui/3.0.0/firebase-ui-auth.css" />
+<script src="https://www.gstatic.com/firebasejs/ui/3.1.0/firebase-ui-auth__{LANGUAGE_CODE}.js"></script>
+<link type="text/css" rel="stylesheet" href="https://www.gstatic.com/firebasejs/ui/3.1.0/firebase-ui-auth.css" />
 ```
 
 where `{LANGUAGE_CODE}` is replaced by the code of the language you want. For example, the French
 version of the library is available at
-`https://www.gstatic.com/firebasejs/ui/3.0.0/firebase-ui-auth__fr.js`. The list of available
+`https://www.gstatic.com/firebasejs/ui/3.1.0/firebase-ui-auth__fr.js`. The list of available
 languages and their respective language codes can be found at [LANGUAGES.md](LANGUAGES.md).
 
 Right-to-left languages also require the right-to-left version of the stylesheet, available at
-`https://www.gstatic.com/firebasejs/ui/3.0.0/firebase-ui-auth-rtl.css`, instead of the default
+`https://www.gstatic.com/firebasejs/ui/3.1.0/firebase-ui-auth-rtl.css`, instead of the default
 stylesheet. The supported right-to-left languages are Arabic (ar), Farsi (fa), and Hebrew (iw).
 
 ### Option 2: npm Module

changelog.txt

@@ -0,0 +1,6 @@
+feature - Adds privacyPolicyUrl config to enable developers to provide privacy policy URL. And shows Term of Service and Privacy Policy link on every screen for GDPR compliance.
+feature - Defines the FirebaseUI externs.
+feature - Defines the typescript definitions.
+feature - Optimizes sign in flow to use updateCurentUser eliminating 2 RPCs in the process.
+fixed - Catches errors that occur due to incompatible versions of firebase.js being used with firebaseui.js.
+fixed - Fixes reCAPTCHA invisible badge which was getting hidden.