Document availability of matchers
Document availability of matchers

Add instructions especially for people who are using ActiveModel or
ActiveRecord outside of Rails and want to use model matchers to test an
arbitrary model.

[ci skip]

guialbuk/shoulda-matchers,plribeiro3000/shoulda-matchers,reacuna/shoulda-matchers,guialbuk/shoulda-matchers,cheshire-cat/shoulda-matchers,cheshire-cat/shoulda-matchers,biow0lf/shoulda-matchers,biow0lf/shoulda-matchers,reacuna/shoulda-matchers,thoughtbot/shoulda-matchers,plribeiro3000/shoulda-matchers,biow0lf/shoulda-matchers,plribeiro3000/shoulda-matchers,thoughtbot/shoulda-matchers,guialbuk/shoulda-matchers,thoughtbot/shoulda-matchers,guialbuk/shoulda-matchers,plribeiro3000/shoulda-matchers,reacuna/shoulda-matchers,reacuna/shoulda-matchers,biow0lf/shoulda-matchers,thoughtbot/shoulda-matchers,cheshire-cat/shoulda-matchers,cheshire-cat/shoulda-matchers