Docs

Added therubyracer gem, otherwise 'rake db:create' gives JS error (due to rails 3.2 assets?).  
Also added a link 'Adding features' using wiki, which needs a wiki file.  
* I cloned bridge_troll.wiki.git (see bridge_troll/wiki 'Git Access' tab for details).  
* Wiki may need a separate pull request?  I haven't added wiki files to github before.  
* 'Adding features' contains what we have so far.  It's a work in progress.  
* Also the relative path link may not work yet (hard to test w/ different repos).