added readme line to set context processors correctly

If you don't have 'pinax_theme_bootstrap.context_processors_theme' in your TEMPLATE_CONTEXT_PROCESSORS setting the site name won't populate templates correctly; similarly, templates now get passed a user context by Django to determine user info, but pinax_theme_bootstrap expects a request context, which the template won't get unless you've set 'django.core.context_processors.request' as a context processor. While the second one should probably just be fixed, for now I've simply updated the readme to reflect the settings.