Add performance doc

Author: tnorling

lib/msal-browser/docs/performance.md

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+# Performance
+
+This document will outline techniques your application can use to improve the performance of acquire tokens using MSAL.js.
+
+## Bypass cloud instance discovery resolution
+
+By default, during the process of retrieving a token, MSAL.js will make a network request to retrieve metadata associated with the various Azure clouds. If you would like to skip this network request, you can provide the required metadata in the configuration of `UserAgentApplication`.
+
+**Important:** It is your application's responsibility to ensure it is using correct, up-to-date cloud instance metadata. Failure to do so may result in your application not working correctly.
+
+**Note:** If you are using B2C or ADFS authorities this document is not applicable. You will need to provide your authority domains to the `auth.knownAuthorities` property instead.
+
+Instructions (AAD Scenarios):
+
+1. Instance Discovery Endpoint: `https://login.microsoftonline.com/common/discovery/instance?api-version=1.1&authorization_endpoint=https://login.microsoftonline.com/common/oauth2/v2.0/authorize`
+2. Make a request to the instance discovery endpoint
+3. Look at the **metadata** field from the response and find your authority domain in one of the `aliases` lists
+4. Provide the **entire** JSON object for each authority you intend to use as a member of an array in the `auth.instanceMetadata` property.
+
+The example shown below shows 5 such JSON objects in the array. If none of the aliases listed match your authority you do not need to include the corresponding object in the array. 
+
+Example:
+
+```js
+const msalInstance = new msal.UserAgentApplication({
+    auth: {
+        instanceMetadata: [
+        {
+          "preferred_network":"login.microsoftonline.com",
+          "preferred_cache":"login.windows.net",
+          "aliases":["login.microsoftonline.com","login.windows.net","login.microsoft.com","sts.windows.net"]
+        },
+        {
+          "preferred_network":"login.partner.microsoftonline.cn",
+          "preferred_cache":"login.partner.microsoftonline.cn",
+          "aliases":["login.partner.microsoftonline.cn","login.chinacloudapi.cn"]
+        },
+        {
+          "preferred_network":"login.microsoftonline.de",
+          "preferred_cache":"login.microsoftonline.de",
+          "aliases":["login.microsoftonline.de"]
+         },
+         {
+           "preferred_network":"login.microsoftonline.us",
+           "preferred_cache":"login.microsoftonline.us",
+           "aliases":["login.microsoftonline.us","login.usgovcloudapi.net"]
+          },
+          {
+            "preferred_network":"login-us.microsoftonline.com",
+            "preferred_cache":"login-us.microsoftonline.com",
+            "aliases":["login-us.microsoftonline.com"]
+          }
+         ]
+    }
+});
+```