Add documentation regarding Bearer Token Authentication set up.

Bug: Issue 16399
Change-Id: I915b69210036928b44bbf33351d23b4c9fec9cda

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index eab5fe6..e256491 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -49,6 +49,16 @@
 To manually trigger replication at runtime, see
 SSH command [start](cmd-start.md).
 
+Enabling Bearer Token Authentication
+------------------------------------
+
+If replicating over HTTP/HTTPS with Bearer Token Authentication, ensure
+the `pull-replication.jar` is copied into the `$GERRIT_SITE/lib`. The reason
+behind is that JGit does not support Bearer Token Authentication and therefore
+pull-replication plugin has implemented a temporal solution to support it,
+and it requires to be loaded in the main Gerrit class-loader, even if it is
+initialised as a plugin.
+
 File `@PLUGIN@.config`
 -------------------------