Update installation instructions in README
Update installation instructions in README

Also:

* Mention RSpec before Minitest (since it's used more often)
* Fix instructions on generating YARD documentation automatically
* Make a new Contributing section
* Use "Shoulda Matchers", not shoulda-matchers
* Add notice that this is the README for the master branch

[ci skip]

taichiman/shoulda-matchers,plribeiro3000/shoulda-matchers,timoschilling/shoulda-matchers,cheshire-cat/shoulda-matchers,raysrashmi/shoulda-matchers,guialbuk/shoulda-matchers,taichiman/shoulda-matchers,biow0lf/shoulda-matchers,timoschilling/shoulda-matchers,guialbuk/shoulda-matchers,reacuna/shoulda-matchers,stanchan/shoulda-matchers,cheshire-cat/shoulda-matchers,awmichel/shoulda-matchers,guialbuk/shoulda-matchers,biow0lf/shoulda-matchers,raysrashmi/shoulda-matchers,timoschilling/shoulda-matchers,cheshire-cat/shoulda-matchers,plribeiro3000/shoulda-matchers,awmichel/shoulda-matchers,AlexVPopov/shoulda-matchers,alejandrogutierrez/shoulda-matchers,stanchan/shoulda-matchers,biow0lf/shoulda-matchers,plribeiro3000/shoulda-matchers,stanchan/shoulda-matchers,raysrashmi/shoulda-matchers,guialbuk/shoulda-matchers,thewatts/shoulda-matchers,reacuna/shoulda-matchers,taichiman/shoulda-matchers,stanchan/shoulda-matchers,thoughtbot/shoulda-matchers,awmichel/shoulda-matchers,AlexVPopov/shoulda-matchers,AlexVPopov/shoulda-matchers,NafaaBout/shoulda-matchers,NafaaBout/shoulda-matchers,alejandrogutierrez/shoulda-matchers,thewatts/shoulda-matchers,alejandrogutierrez/shoulda-matchers,reacuna/shoulda-matchers,raysrashmi/shoulda-matchers,plribeiro3000/shoulda-matchers,thoughtbot/shoulda-matchers,alejandrogutierrez/shoulda-matchers,biow0lf/shoulda-matchers,cheshire-cat/shoulda-matchers,reacuna/shoulda-matchers,NafaaBout/shoulda-matchers,thoughtbot/shoulda-matchers,taichiman/shoulda-matchers,AlexVPopov/shoulda-matchers,awmichel/shoulda-matchers,thewatts/shoulda-matchers,thoughtbot/shoulda-matchers,timoschilling/shoulda-matchers,NafaaBout/shoulda-matchers,thewatts/shoulda-matchers