docs: add high-level description of SSH repository access In addition to the existing technical documentation about SSH repository access, add some high-level info about what this means. (some editing by Thomas De Schampheleire)

[tox]
minversion = 1.8
envlist = py27-pytest

[testenv]
setenv =
    PYTHONHASHSEED = 0
deps =
    -r{toxinidir}/dev_requirements.txt
    python-ldap
    python-pam
commands =
    pytest: py.test {posargs}