Add example dev config file, config file docs.

docs/using.rst

@@ -151,6 +151,27 @@ The ``letsencrypt`` commandline tool has a builtin help:
    ./venv/bin/letsencrypt --help
 
 
+Configuration file
+------------------
+
+It is possible to specify configuration file with
+``letsencrypt --config cli.ini`` (or shorter ``-c cli.ini``). For
+instance, if you are a contributor, you might find the following
+handy:
+
+.. include:: ../examples/dev-cli.ini
+   :code: ini
+
+By default, the following locations are searched:
+
+- ``/etc/letsencrypt/cli.ini``
+- ``$XDG_CONFIG_HOME/letsencrypt/cli.ini`` (or
+  ``~/.config/letsencrypt/cli.ini`` if ``$XDG_CONFIG_HOME`` is not
+  set).
+
+.. keep it up to date with constants.py
+
+
 .. _Augeas: http://augeas.net/
 .. _M2Crypto: https://github.com/M2Crypto/M2Crypto
 .. _SWIG: http://www.swig.org/

examples/dev-cli.ini

@@ -0,0 +1,17 @@
+# This is an example configuration file for developers
+config-dir = /tmp/le/conf
+work-dir = /tmp/le/conf
+logs-dir = /tmp/le/logs
+
+# make sure to use a valid email and domains!
+email = foo@example.com
+domains = example.com
+
+text = True
+agree-eula = True
+debug = True
+# Unfortunately, it's not possible to specify "verbose" multiple times
+# (correspondingly to -vvvvvv)
+verbose = True
+
+authenticator = standalone