Access token forwarding through nginx auth request (#68)

* Access token forwarding through nginx auth request

Related to #420.

(cherry picked from commit b138872beaaa7f47d43a1c0fef11a67f57e61eff)
Signed-off-by: David Holsgrove <david.holsgrove@biarri.com>

* Improved documentation for auth request token

(cherry picked from commit 6fab314f7203f4d652bb34247abb4e7cb497c41d)
Signed-off-by: David Holsgrove <david.holsgrove@biarri.com>

* Update README.md

Example should set header as `X-Access-Token`

Co-Authored-By: davidholsgrove <davidholsgrove@users.noreply.github.com>

* Update Changelog to reference https://github.com/pusher/oauth2_proxy/pull/68

* Fix Changelog message location