Add a note about window.JSON dependency

Raven discards errors when window.JSON is not present. Since IE support for window.JSON is deponent on documents mode even on recent versions, it is better to let the user know about it

Author: j-manu

docs/install/index.rst

@@ -45,3 +45,8 @@ We also provide a way to deploy Raven via `bower
 
 Also note that the file is uncompresed but is ready to pass to any decent JavaScript compressor like `uglify <https://github.com/mishoo/UglifyJS2>`_.
 
+Requirements
+~~~~~~~~~~~~
+
+Raven expects the browser to provide `window.JSON` and `window.JSON.stringify`. In Internet Explorer 8+ these are available in `standards mode <http://msdn.microsoft.com/en-us/library/cc288325(VS.85).aspx>`_.
+You can also use `json2.js <https://github.com/douglascrockford/JSON-js>`_ to provide the JSON implementation in browsers/modes which doesn't support native JSON