Add Define A Custom RSpec Matcher as a ruby til

ruby/define-a-custom-rspec-matcher.md

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+# Define A Custom RSpec Matcher
+
+You can define your own custom RSpec matchers. This is a great way to keep
+your spec tight and readable. A custom, domain-specific matcher can convey
+more intent in a single line than several lines of built-in matchers.
+
+Here is a matcher to check if something is in a range:
+
+```ruby
+require 'rspec/expectations'
+
+RSpec::Matchers.define :be_betwen do |lower, upper|
+  match do |operand|
+    expect(operand).to be >= lower
+    expect(operand).to be <= upper
+  end
+end
+
+describe MyThing do
+  it "has a value between 0 and 10" do
+    thing = MyThing.new
+
+    expect(thing.value).to be_between(0, 10)
+  end
+end
+```
+
+By requiring `rspec/expectations` we are able to define a matcher that takes
+0 or more arguments (in our case, `lower` and `upper`) and then make
+assertions with them and the `expect` value.
+
+[source](https://relishapp.com/rspec/rspec-expectations/v/2-4/docs/custom-matchers/define-matcher)