Added FAQ about using shared mailboxes with Office365

Improves docs for issue #1770

FAQ.md

@@ -37,6 +37,7 @@
 * [Why doesn't ImapFolder.MoveTo() move the message out of the source folder?](#imap-move-does-not-move)
 * [How can I mark messages as read using IMAP?](#imap-mark-as-read)
 * [How can I re-synchronize the cache for an IMAP folder?](#imap-folder-resync)
+* [How can I login using a shared mailbox in Office365?](#office365-shared-mailboxes)
 
 ### SmtpClient
 
@@ -1731,6 +1732,31 @@ static void ResyncFolder (ImapFolder folder, List<CachedMessageInfo> cache, ref
 }
 ```
 
+### <a href="office365-shared-mailboxes">Q: How can I login using a shared mailbox in Office365?</a>
+
+```csharp
+var result = await GetPublicClientOAuth2CredentialsAsync ("IMAP", "[email protected]");
+
+// Note: We always use result.Account.Username instead of `Username` because the user may have selected an alternative account.
+var oauth2 = new SaslMechanismOAuth2 (result.Account.Username, result.AccessToken);
+
+using (var client = new ImapClient ()) {
+    await client.ConnectAsync ("outlook.office365.com", 993, SecureSocketOptions.SslOnConnect);
+    await client.AuthenticateAsync (oauth2);
+
+    // ...
+
+    await client.DisconnectAsync (true);
+}
+```
+
+Notes:
+
+1. The `GetPublicClientOAuth2CredentialsAsync()` method used in this example code snippet can be found in the
+[ExchangeOAuth2.md](ExchangeOAuth2.md#desktop-and-mobile-applications) documentation.
+2. Some users have reported that they need to use `"[email protected]\\sharedMailboxName"` as their
+username instead of `"[email protected]"`.
+
 ## SmtpClient
 
 ### <a id="smtp-sent-folder">Q: Why doesn't the message show up in the "Sent Mail" folder after sending it?</a>