Brew directory and README improvements

* Add README information on where to find dictionaries and how to install them
* Support homebrew suggested directories for dictionaries (as specified in their caveats pasted below)

> Dictionary files (*.aff and *.dic) should be placed in ~/Library/Spelling/ or /Library/Spelling/.  Homebrew itself provides no dictionaries for Hunspell, but you can download compatible dictionaries from other sources, such as https://wiki.openoffice.org/wiki/Dictionaries .