Refs #332: Add note about db backend
Refs #332: Add note about db backend

Add note suggested by copelco about the db backend not likely
to be used in production.

Write your commit message in the imperative: "Fix bug" and not "Fixed bug"
or "Fixes bug."  This convention matches up with commit messages generated
by commands like git merge and git revert.

lsgunth/rapidsms,peterayeni/rapidsms,ehealthafrica-ci/rapidsms,catalpainternational/rapidsms,eHealthAfrica/rapidsms,caktus/rapidsms,lsgunth/rapidsms,peterayeni/rapidsms,peterayeni/rapidsms,lsgunth/rapidsms,caktus/rapidsms,caktus/rapidsms,catalpainternational/rapidsms,ehealthafrica-ci/rapidsms,catalpainternational/rapidsms,catalpainternational/rapidsms,peterayeni/rapidsms,ehealthafrica-ci/rapidsms,eHealthAfrica/rapidsms,lsgunth/rapidsms,eHealthAfrica/rapidsms