Add note about requirements lower bounds

Since lower constraints job was removed the bounds are not actually
tested. Add a note to explain the bounds are maintained on best effort
basis.

Change-Id: I69dcba9227122592a07c77e2caeaaf855df6c480

Author: kajinamit

requirements.txt

@@ -1,3 +1,7 @@
+# Requirements lower bounds listed here are our best effort to keep them up to
+# date but we do not test them so no guarantee of having them all correct. If
+# you find any incorrect lower bounds, let us know or propose a fix.
+
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 PrettyTable>=0.7.2 # BSD
 requests>=2.14.2 # Apache-2.0