docs: add credential helper for outlook and gmail in OAuth list of helpers

This commit adds the `git-credential-outlook` and `git-credential-gmail`
helpers to the list of OAuth helpers.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: AdityaGarg8

Documentation/gitcredentials.adoc

@@ -133,6 +133,10 @@ Popular helpers with OAuth support include:
 
     - https://github.com/hickford/git-credential-oauth[git-credential-oauth] (cross platform, included in many Linux distributions)
 
+    - https://github.com/AdityaGarg8/git-credential-email[git-credential-gmail] (cross platform, dedicated helper to authenticate Gmail accounts for linkgit:git-send-email[1])
+
+    - https://github.com/AdityaGarg8/git-credential-email[git-credential-outlook] (cross platform, dedicated helper to authenticate Microsoft Outlook accounts for linkgit:git-send-email[1])
+
 CREDENTIAL CONTEXTS
 -------------------