Update developer docs to describe motivation behind vendored dependencies. Ref #1781.

docs/developer-guide.txt

@@ -137,3 +137,17 @@ To build the docs locally, use tox::
 
 .. _Sphinx: http://www.sphinx-doc.org/en/master/
 .. _published documentation: https://setuptools.readthedocs.io/en/latest/
+
+---------------------
+Vendored Dependencies
+---------------------
+
+Setuptools has some dependencies, but due to `bootstrapping issues
+<https://github.com/pypa/setuptools/issues/980>`, those dependencies
+cannot be declared as they won't be resolved soon enough to build
+setuptools from source. Eventually, this limitation may be lifted as
+PEP 517/518 reach ubiquitous adoption, but for now, Setuptools
+cannot declare dependencies other than through
+``setuptools/_vendor/vendored.txt`` and
+``pkg_reosurces/_vendor/vendored.txt`` and refreshed by way of
+``paver update_vendored`` (pavement.py).