[doc] Note lack of constraints is a choice

Change-Id: I1eb824e155a6b868d7dd2b4a1308492c9e86abc2

tox.ini

@@ -6,6 +6,7 @@ envlist = py{34,27,35},{debug,py,py34,py27,py35}-{mongodb,mysql,postgresql,elast
 [testenv]
 deps = .[mongo,mysql,postgresql,gnocchi]
        -r{toxinidir}/test-requirements.txt
+# NOTE(tonyb): This project has chosen to *NOT* consume upper-constraints.txt
 install_command = pip install -U {opts} {packages}
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}