Documentation: vm, add hugetlbfs reservation overview

Adding a brief overview of hugetlbfs reservation design and
implementation as an aid to those making code modifications in this
area.

Link: http://lkml.kernel.org/r/1491586995-13085-1-git-send-email-mike.kravetz@oracle.comSigned-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/00-INDEX

@@ -12,6 +12,8 @@ highmem.txt
 	- Outline of highmem and common issues.
 hugetlbpage.txt
 	- a brief summary of hugetlbpage support in the Linux kernel.
+hugetlbfs_reserv.txt
+	- A brief overview of hugetlbfs reservation design/implementation.
 hwpoison.txt
 	- explains what hwpoison is
 idle_page_tracking.txt