doc: gitcredentials: link to helper list

Link to community list of credential helpers. This is useful information
for users.

Describe how OAuth credential helpers work. OAuth is a user-friendly
alternative to personal access tokens and SSH keys. Reduced setup cost
makes it easier for users to contribute to projects across multiple
forges.

Signed-off-by: M Hickford <mirth.hickford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: hickford

Documentation/gitcredentials.txt

@@ -104,6 +104,17 @@ $ git help credential-foo
 $ git config --global credential.helper foo
 -------------------------------------------
 
+=== Available helpers
+
+The community maintains a comprehensive list of Git credential helpers at
+https://git-scm.com/doc/credential-helpers.
+
+=== OAuth
+
+An alternative to inputting passwords or personal access tokens is to use an
+OAuth credential helper. Initial authentication opens a browser window to the
+host. Subsequent authentication happens in the background. Many popular Git
+hosts support OAuth.
 
 CREDENTIAL CONTEXTS
 -------------------