Add example and update readme

Author: glesica

README.md

@@ -40,6 +40,7 @@ For more information take a look at the [quick start](#quick-start), the [exampl
       * [line-count](#line-count)
       * [not-contains](#not-contains)
       * [xml](#xml)
+      * [file](#file)
     - [stderr](#stderr)
     - [skip](#skip)
   + [Config](#user-content-config-config)
@@ -530,6 +531,21 @@ cat some.xml:
 </book>
 ```
 
+##### file
+
+`file` is a file path, relative to the working directory that will have
+its entire contents matched against the command output. Other than
+reading from a file this works the same as [exactly](#exactly).
+
+The example below will always pass.
+
+```yaml
+output should match file:
+  command: cat output.txt
+  stdout:
+    file: output.txt
+```
+
 #### stderr
 
 See [stdout](#stdout) for more information.

examples/_fixtures/output.txt

@@ -0,0 +1,2 @@
+line one
+line two

examples/commander.yaml

@@ -24,4 +24,9 @@ tests:
   it should skip:
     command: echo "I should be skipped"
     stdout: I should be skipped
-    skip: true
+    skip: true
+
+  it should match file output:
+    command: printf "line one\nline two"
+    stdout:
+      file: _fixtures/output.txt