Add readme.

README.md

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+`HSCountingInputStream` is a simple example of an `NSInputStream` subclass
+that works around the bugs you'll run into if you try to pass such a subclass
+to NSURLRequest or CFHTTPMessageRef. See [my blog](http://bjhomer.blogspot.com/2011/04/subclassing-nsinputstream.html)
+for more information on the gory details of how it works.
+
+Note:
+
+This sample uses a few language features only available if you're
+using Xcode 4 and the LLVM Compiler 2.0, such as instance variables defined
+in the @implementation block. It also relies on the modern runtime to
+synthesize the instance variables.
+I used these features because they're cool and I wanted people to be aware
+of them.
+
+So if it won't compile for you, that's probably why. It shouldn't
+be hard to make the necessary changes to make it work for x86 or GCC 4.2,
+if that's what you need to do.