Bug 16945: PatronSelfRegistration: Add note about setting PatronSelfRegistrationDefau...

Bug 16945: PatronSelfRegistration: Add note about setting PatronSelfRegistrationDefaultCategory

In OPAC, patron self registration works only when
PatronSelfRegistrationDefaultCategory is set to a valid patron category.

This patch adds a note for
PatronSelfRegistration on Home > Administration > System preferences.

To test:
- Apply patch
- In staff client, search system preferences for PatronSelfRegistration
- Verify that note appears with PatronSelfRegistration

(Amended to fix typo, see comment #3, 2016-07-21 mv)

Signed-off-by: Claire Gravely <c.gravely@arts.ac.uk>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>