Add credential helpers for Gmail, Outlook and Yahoo

Author: AdityaGarg8

content/doc/credential-helpers.html

@@ -33,6 +33,9 @@ <h2>OAuth</h2>
   <ul>
     <li><a href="https://github.com/git-ecosystem/git-credential-manager">Git Credential Manager</a>: included with Git for Windows. Supports multiple credential stores.</li>
     <li><a href="https://github.com/hickford/git-credential-oauth">git-credential-oauth</a>: included in many Linux distributions.</li>
+    <li><a href="https://github.com/AdityaGarg8/git-credential-email">git-credential-gmail</a>: cross platform, can be used to authenticate Gmail accounts for <a href="https://git-scm.com/docs/git-send-email">git send-email</a>.</li>
+    <li><a href="https://github.com/AdityaGarg8/git-credential-email">git-credential-outlook</a>: cross platform, can be used to authenticate Microsoft Outlook accounts for <a href="https://git-scm.com/docs/git-send-email">git send-email</a>.</li>
+    <li><a href="https://github.com/AdityaGarg8/git-credential-email">git-credential-yahoo</a>: cross platform, can be used to authenticate Yahoo accounts for <a href="https://git-scm.com/docs/git-send-email">git send-email</a>.</li>
   </ul>
 
   <h2>Storage specific</h2>