Allow easy templates overriding

The spirit is to have an easy way to provide an (out of coin git) dir which
contains <some-isp> specific templates.

<some-isp> is then free to manage its custom templates in an external git or
the way it wants.

- Add an EXTRA_TEMPLATE_DIRS setting
- Add the documentation to explain how to override templates

README.md

@@ -198,6 +198,44 @@ can set an email address in the crontab:
 
     MAILTO=tresorier@myisp.fr
 
+Customizing templates
+---------------------
+
+You may want to override some of the (HTML or email) templates to better suit
+your structure needs.
+
+Coin allows you to have a folder of custom templates that will contain your
+templates, which gets loaded prior to coin builtins.
+
+The templates you may override are stored in several places :
+
+- `coin/templates/`
+- `coin/<app_name>/templates/` folders (iterating every app)
+
+### Do once (setup)
+
+- Create a folder dedicated to your custom templates (*hint: outside of coin git
+  tree is better*).
+- Register that folder in the `EXTRA_TEMPLATE_DIRS` settings. Ex:
+
+    EXTRA_TEMPLATE_DIRS = ('/home/coin/my-folder/templates',)
+
+### For each template you want to override
+
+Copy the template you want to override to the right place in your custom
+ folder (that's the hard part, see the example).
+
+*Example*
+
+Say we want to override the temalet located at
+`coin/members/templates/members/emails/call_for_membership_fees.html` and we
+set `EXTRA_TEMPLATE_DIRS = ('/home/coin/my-folder/templates',)` in settings.
+
+Then  make a copy of the template file (and customize it) at
+`/home/coin/my-folder/templates/members/emails/call_for_membership_fees.html`
+
+Good to go :-)
+
 More information
 ================
 

coin/settings.py

@@ -137,6 +137,8 @@ TEMPLATE_DIRS = (
     os.path.join(PROJECT_PATH, 'templates/'),
 )
 
+EXTRA_TEMPLATE_DIRS = tuple()
+
 INSTALLED_APPS = (
     'django.contrib.auth',
     'django.contrib.contenttypes',
@@ -250,3 +252,5 @@ try:
     from settings_local import *
 except ImportError:
     pass
+
+TEMPLATE_DIRS = EXTRA_TEMPLATE_DIRS + TEMPLATE_DIRS