Bug 28412: Add information about supported authentication methods

Bug 28412: Add information about supported authentication methods

This patch adds an initial text explaining the valid authentication
mechanisms Koha has for the API.

My first approach included information about the controlling sysprefs,
but I decided to remove it, as it is not usual for API consumers to be
presented such detailed internal information which is probably
confusing. We could add a link to the manual, but that's not the point I
think.

I've added an entry about error messages and how we thougth about them.
It is copied from Docker Engine's API docs.

This is my 2 cents.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>