Improve yum_requirements.txt documentation

[Lnaden]

As per discussion on conda-forge/staged-recipes#299 (comment), it would be helpful to see better documentation on how to use yum_requirements.txt to install additional yum packages, and the warnings about how those packages will be carried on to any packages which depend on it.

Currently the only mention of it is on the conda-smithy documentation in passing and its up to the user to determine its usage from example in other conda-forge repos.

[croth1]

I added a general message discouraging the use of yum_dependencies.txt and two exception to the general rule. See: http://conda-forge.org/docs/maintainer/knowledge_base.html#yum-requirements-txt