Add FAQ entry for subprojects.

docs/faq.rst

@@ -80,6 +80,23 @@ It looks something along the lines of:
     {%- endfor -%}
     ">
 
+How do I host multiple projects on one CNAME?
+---------------------------------------------
+
+We support the concept of Subprojects.
+If you add a subproject to a project,
+that documentation will also be served under the parent project's subdomain.
+
+For example,
+Kombu is a subproject of celery,
+so you can access it on the `celery.readthedocs.org` domain:
+
+http://celery.readthedocs.org/projects/kombu/en/latest/
+
+This also works the same for CNAME's:
+
+http://docs.celeryproject.org/projects/kombu/en/latest/
+
 Where do I need to put my docs for RTD to find it?
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