Describe the order of precedence for configuration in README

[mark-dce]

It would be nice for the readme to clearly describe the order of precedence for configuration options. Particularly since it now looks like solr_wrapper will read multiple config files simultaneously ;)

I just ran a recent version of solr wrapper and it says it's loading configuration from two files

$ bin/rails ci
Loading configuration from /home/vagrant/laevigata/.solr_wrapper.yml
Loading configuration from /home/vagrant/laevigata/.solr_wrapper
solr-6.5.0.zip: ...

I didn't create either file so I'm assuming both are created by generators called by the hyrax template application following the instructions here https://github.com/projecthydra-labs/hyrax#creating-a-hyrax-based-app. Based on the contents, it looks like .solr_wrapper.yml is put there by a blacklight generator and .solr_wrapper by some hydra/hyrax generator.

$ bundle list solr_wrapper
/home/vagrant/.rvm/gems/ruby-2.3.4@laevigata/gems/solr_wrapper-0.23.0
``