TEMPLATE_LOADERS was defined after loading local_settings.py and was sqashing
any attempt to customize the template loaders.
This patch adds CACHED_TEMPLATE_LOADERS, ADD_TEMPLATE_LOADERS and a doc
defining how to use them in order to ease customization process.
Change-Id: I2544529ee965ef01c6ac4973056801ebee50be6d
Closes-Bug: #1568764