fixes some typos, adds changelog document

CHANGELOG.md

@@ -0,0 +1,11 @@
+This document details change between individual versions.
+
+For instructions on how to upgrade from one version to another, please see the dedicated UPGRADING document.
+
+0.12 (tbd)
+----------
+- adds, and exposes SerializationContext/DeserializationContext
+
+0.11 (2013-01-29)
+-----------------
+Initial Release

UPGRADING.md

@@ -13,7 +13,7 @@ From 0.11 to 0.12
 
     After:
 
-        $serializer->serialize($data, 'json', Context::create()->setVersion(1));
+        $serializer->serialize($data, 'json', SerializationContext::create()->setVersion(1));
 
 - All visit??? methods of the VisitorInterface, now require a third argument, the Context; the context is for example
   passed as an additional argument to handler, exclusion strategies, and also available in event listeners.