Add blank line in README to improve readability

Although it's not necessary for markdown formatting, other headings are
separated from their subsequent paragraph(s) by a blank line and I think
this makes the unformatted version easier to read.

README.md

@@ -8,6 +8,7 @@
 * Supported by many other test frameworks.
 
 ### Intended Usage
+
 Mocha is intended to be used in unit tests for the [Mock Object](http://xunitpatterns.com/Mock%20Object.html) or [Test Stub](http://xunitpatterns.com/Test%20Stub.html) types of [Test Double](http://xunitpatterns.com/Test%20Double.html), not the [Fake Object](http://xunitpatterns.com/Fake%20Object.html) or [Test Spy](http://xunitpatterns.com/Test%20Spy.html) types. Although it would be possible to extend Mocha to allow the implementation of fakes and spies, we have chosen to keep it focused on mocks and stubs.
 
 ### Installation