Add info about building the v1 api-ref

Because the Image API is removed in Rocky, its API Reference has
been deleted.  Add information about why it's gone and how to build
it to the current API Reference.

Closes-bug: #1762387

Change-Id: Ic215e7367b49df1ac837a4a1fa56effa7d9f7a61

api-ref/source/versions/index.rst

@@ -77,3 +77,46 @@ Version History
 **Bexar changes**
 
 - version 1.0 is CURRENT
+
+What happened to the v1 API?
+****************************
+
+The Image Service API version 1 was DEPRECATED in the OpenStack Newton release
+and removed during the development cycle for the Rocky release.  The last
+OpenStack release containing the Image Service API v1 was the Queens release.
+
+The source files for the Image Service API Reference are contained in the
+OpenStack Glance source code repository.  The files for the version 1 reference
+are no longer in the current development branch, but they may still be found
+in the stable branches in the repository.
+
+If you would like to consult the Image Service API version 1 Reference, you can
+check out a stable branch from the repository, build it locally, and use a web
+browser to read the generated HTML files.
+
+Building the API Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You'll need to have the following installed on your system:
+
+* python
+* git
+* tox
+
+Then:
+
+1. Go to the Glance repository mirror on GitHub:
+   https://github.com/openstack/glance
+
+2. Clone the repository to your local system.
+
+3. Checkout the **stable/queens** branch of glance.
+
+4. From the root directory, use tox to build the api-reference:
+
+   ``tox -e api-ref``
+
+5. The HTML version of the Image Service API Reference will be located in the
+   ``api-ref/build/html`` directory.  Use your browser to open the
+   ``index.html`` file in that directory and you'll be able to browse the API
+   Reference.