Refactor Dockerfile location

Link to contrib/docker

Link to docker/README.md

Link to contrib/docker

wrap at 80ish

Mention the word "newsfragment" in CONTRIBUTING.rst

This addresses #3224

Clean up handling of errors from outbound requests

Factor out exception handling in federation_client

Synapse 0.33.1 (2018-08-02)
===========================

SECURITY FIXES
--------------

- Fix a potential issue where servers could request events for rooms they have not joined. (`#3641 <https://github.com/matrix-org/synapse/issues/3641>`_)
- Fix a potential issue where users could see events in private rooms before they joined. (`#3642 <https://github.com/matrix-org/synapse/issues/3642>`_)

Check room visibility for /event/ requests

Make sure that the user has permission to view the requeseted event for
/event/{eventId} and /room/{roomId}/event/{eventId} requests.

Also check that the event is in the given room for
/room/{roomId}/event/{eventId}, for sanity.

Validation for events/rooms in fed requests

Since we're about to look up the events themselves anyway, we can skip the
extra db queries here.

When we get a federation request which refers to an event id, make sure that
said event is in the room the caller claims it is in.

(patch supplied by @turt2live)

Split out DB writes in federation handler

Initial impl of capping MAU

return 404 if room not found