[docs] Fuel development docs for Nailgun does not have instruction to set up nailgun extensions

[Vladimir Kuklin]

Follow https://docs.fuel-infra.org/fuel-dev/develop/nailgun/development/env.html from master code (SHA) 0a3a744bd5aa8889805f62f267531af4ff7a3bdb

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ProgrammingError: (psycopg2.ProgrammingError) relation "volume_manager_node_volumes" does not exist

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/280717

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/280717
Committed: https://git.openstack.org/cgit/openstack/fuel-web/commit/?id=5b1440593a5b0018f4e4c779774bff884b0b6422
Submitter: Jenkins
Branch: master

commit 5b1440593a5b0018f4e4c779774bff884b0b6422
Author: Sylwester Brzeczkowski <email address hidden>
Date: Tue Feb 16 15:08:53 2016 +0100

    Update Nailgun documentation about extensions

    Added a step in `Preparing Development Environment` about installing
    Nailgun in developers mode. Thanks to that extensions will be
    discovered.

    Change-Id: Ieb853bcea2a5bc7448cbe59b3c865f566d048b9c
    Closes-Bug: #1544534
    Implements: blueprint stevedore-extensions-discovery

[Michele Fagan (michelefagan)]

Removed area-docs as fix commit is for dev docs

[Alisa Tselovalnikova (atselovalnikova)]

A step in `Preparing Development Environment` about installing Nailgun is added in https://docs.fuel-infra.org/fuel-dev/develop/nailgun/development/env.html.