Add Git credential config docs (#1683)

components/external_components.rst

@@ -42,6 +42,8 @@ Configuration variables:
 
   - **url** (**Required**, url): HTTP git repository url. See :ref:`external-components_git`.
   - **ref** (*Optional*, string): Git ref (branch or tag). If not specified the default branch is used.
+  - **username** (*Optional*, string): Username for the Git server, if one is required
+  - **password** (*Optional*, string): Password for the Git server, if one is required
 
   local options:
 
@@ -182,6 +184,12 @@ folder and components will then be loaded from this local copy. The local path o
 varies per repository name and ref name, so repositories with different refs are considered different
 repositories and updated independently.
 
+If required, you can supply a username and password to use when authenticating with the remote git
+server using the ``username`` and ``password`` fields. This is most useful when combined with the
+``!secret``  feature, to load the values in from a ``secrets.yaml`` file. This is not a comprehensive
+security measure; your username and password will necessarily be stored in clear text within the
+``.esphome`` directory.
+
 .. _external-components_refresh:
 
 Refresh