update README with Spring config

[onebree]

I hope the way I phrased everything is okay. As mentioned in issue #381 I set two configurations to false in my environment files. Such settings were also recommended by Rails. And although Spring preloads your environment, I find that still disabling eager_load helps Spring cooperate with SimpleCov

ALSO I know the contributing doc says to write tests for any changes, but since this is just the README, I do not think it is necessary. I can provide my own stats if needed (running my own suite).

DEPRECATION WARNING: The configuration option `config.serve_static_assets` has been renamed to
`config.serve_static_files` to clarify its role (it merely enables serving everything in the
`public` folder and is unrelated to the asset pipeline). The `serve_static_assets` alias will be
removed in Rails 5.0. Please migrate your configuration files accordingly. (called from block in
<top (required)> at /var/www/rails/hosted-fork/hpbxgui/config/environments/development.rb:15)

config.eager_load is set to nil. Please update your config/environments/*.rb files accordingly:

  * development - set it to false
  * test - set it to false (unless you use a tool that preloads your test environment)
  * production - set it to true

[sferik]

Thanks for this update!