Update customrenderers.rst

The renderer implementation now offers overriding content type of response and does not fail if rows and headers are not passed to the renderer. Varying attributes of response like settings a filename is done in view code as considered best practice in pyramid docs. Both code samples are now more focused to the task, the view code in this recipe should not care much about the filename pattern. Fixed wrong code indentation as well. Added important links to pyramid documentation.
Merge pull request #116 from saschagottfried/patch-1

Update customrenderers.rst