Nobody really bothered with the default AI much before because silverfish was so much better with little effort. The default AI seems to allow much more customization though so people could potentially write good deck rules...

All you need to do to get started is to open any of the deck rule files in \HearthRanger\DeckRuleSet with something capable of editing XLSX (excel) files. I think you can just use Google Docs or Microsoft OneDrive to edit them in your browser but I haven't tested if the downloaded files work or not. Inside the deckrule spreadsheets there are several pages (1 for mulligans, 1 for when to play cards, etc.) and the last few have info on how to write your own rules.