Add link to readme; add few expectations to the tests

Author: pitr-ch

README.md

@@ -67,6 +67,7 @@ Collection classes that were originally part of the (deprecated) `thread_safe` g
 
 * [Array](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Array.html) A thread-safe subclass of Ruby's standard [Array](http://ruby-doc.org/core-2.2.0/Array.html).
 * [Hash](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Hash.html) A thread-safe subclass of Ruby's standard [Hash](http://ruby-doc.org/core-2.2.0/Hash.html).
+* [Set](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Set.html) A thread-safe subclass of Ruby's standard [Set](http://ruby-doc.org/stdlib-2.4.0/libdoc/set/rdoc/Set.html).
 * [Map](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Map.html) A hash-like object that should have much better performance characteristics, especially under high concurrency, than `Concurrent::Hash`.
 * [Tuple](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Tuple.html) A fixed size array with volatile (synchronized, thread safe) getters/setters.
 

spec/concurrent/array_spec.rb

@@ -13,6 +13,7 @@ module Concurrent
           end
         end
       end.map(&:join)
+      expect(ary).to be_empty
     end
 
     describe '#slice' do

spec/concurrent/hash_spec.rb

@@ -7,11 +7,12 @@ module Concurrent
         Thread.new do
           1000.times do |j|
             hsh[i * 1000 + j] = i
-            hsh[i * 1000 + j]
-            hsh.delete(i * 1000 + j)
+            expect(hsh[i * 1000 + j]).to eq(i)
+            expect(hsh.delete(i * 1000 + j)).to eq(i)
           end
         end
       end.map(&:join)
+      expect(hsh).to be_empty
     end
   end
 end

spec/concurrent/set_spec.rb

@@ -1,18 +1,20 @@
-require 'set' 
+require 'set'
 module Concurrent
-  describe Set do 
+  RSpec.describe Set do
     let!(:set) { described_class.new }
 
     it 'concurrency' do
       (1..THREADS).map do |i|
         Thread.new do
-           1000.times do |j|
-            set << i 
-            set.empty?
-            set.delete(i)
-           end
+          1000.times do
+            v = i
+            set << v
+            expect(set).not_to be_empty
+            set.delete(v)
+          end
         end
       end.map(&:join)
+      expect(set).to be_empty
     end
   end
-end 
+end