mdadm: move documentation to folder

Move documentation text files to directory.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>

diff --git a/external-reshape-design.txt b/documentation/external-reshape-design.txt
similarity index 100%
rename from external-reshape-design.txt
rename to documentation/external-reshape-design.txt

diff --git a/mdadm.conf-example b/documentation/mdadm.conf-example
similarity index 100%
rename from mdadm.conf-example
rename to documentation/mdadm.conf-example

diff --git a/mdmon-design.txt b/documentation/mdmon-design.txt
similarity index 100%
rename from mdmon-design.txt
rename to documentation/mdmon-design.txt