Add documentation on pip's role as an installer

Author: pfmoore

docs/html/topics/index.md

@@ -21,4 +21,5 @@ repeatable-installs
 secure-installs
 vcs-support
 python-option
+workflow
 ```

docs/html/topics/workflow.md

@@ -0,0 +1,21 @@
+# Pip is not a workflow tool
+
+The core purpose of pip is to *install packages*. Whilst installing packages is
+an important part of most Python development workflows, it is only one part.
+Managing a development workflow is, in itself, a complex task and one where
+there are many views on the "correct approach".
+
+Pip has a number of features which make it useful in development workflows - for
+example, the ability to install the current project via `pip install .`,
+editable installs, and requirements files. However, there is no intention that
+pip will manage the workflow as a whole.
+
+## The role of `ensurepip`
+
+Pip is available in a standard Python installation, via the `ensurepip` stdlib
+module. This provides users with an "out of the box" installer, which can be
+used to gain access to all of the various tools and libraries available on PyPI.
+In particular, this includes a number of workflow tools.
+
+This "bootstrapping" mechanism was proposed (and accepted) in [PEP
+453](https://www.python.org/dev/peps/pep-0453/).