Add usage assumptions about user types

Ref. pypi#2794.

docs/application.rst

@@ -28,6 +28,33 @@ pattern. This method of handling URLs may be unfamiliar to developers used to
 other web frameworks, such as Django or Flask. `This article`_ has a helpful
 discussion of the differences between URL dispatch and traversal in Pyramid.
 
+Usage assumptions and concepts
+------------------------------
+
+See `PyPI help <https://pypi.org/help/#packages>`_ to understand
+projects, releases, and packages.
+
+Warehouse is specifically the codebase for the official Python Package
+Index, and thus focuses on architecture and features for PyPI and Test
+PyPI. People and groups who want to run their own package indexes
+usually use other tools, like `devpi
+<https://pypi.org/project/devpi-server/>`_.
+
+Warehouse serves three main classes of users:
+
+1. *People who are not logged in.* This accounts for the majority of
+   browser traffic and all API download traffic.
+2. *Owners/maintainers of one or more projects.* This accounts for
+   almost all writes. A user must create and use a PyPI account to
+   maintain or own a project, and there is no particular functionality
+   available to a logged-in user other than to manage projects they
+   own/maintain. As of March 2018, PyPI had about 270,000 users, and
+   Test PyPI had about 30,000 users.
+3. *PyPI application administrators*, e.g., Ernest W. Durbin III,
+   Dustin Ingram, and Donald Stufft, who add classifiers, ban
+   spam/malware projects, help users with account recovery, and so
+   on. There are under ten such admins.
+
 Since reads are *much* more common than writes (much more goes out than
 goes in), we try to cache as much as possible. This is a big reason
 that, although we have supported localization in the past, `we currently