docs: filesystems: convert romfs.txt to ReST

- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d2cc83e7cd6de63c793ccd3f2588ea40f7f1e764.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -85,5 +85,6 @@ Documentation for filesystem implementations.
    qnx6
    ramfs-rootfs-initramfs
    relay
+   romfs
    virtiofs
    vfat

Documentation/filesystems/romfs.rst

@@ -1,4 +1,8 @@
-ROMFS - ROM FILE SYSTEM
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+ROMFS - ROM File System
+=======================
 
 This is a quite dumb, read only filesystem, mainly for initial RAM
 disks of installation disks.  It has grown up by the need of having
@@ -51,7 +55,7 @@ the 16 byte padding for the name and the contents, also 16+14+15 = 45
 bytes.  This is quite rare however, since most file names are longer
 than 3 bytes, and shorter than 15 bytes.
 
-The layout of the filesystem is the following:
+The layout of the filesystem is the following::
 
  offset	    content
 
@@ -84,7 +88,7 @@ the source.  This algorithm was chosen because although it's not quite
 reliable, it does not require any tables, and it is very simple.
 
 The following bytes are now part of the file system; each file header
-must begin on a 16 byte boundary.
+must begin on a 16 byte boundary::
 
  offset	    content
 
@@ -114,7 +118,9 @@ file is user and group 0, this should never be a problem for the
 intended use.  The mapping of the 8 possible values to file types is
 the following:
 
+==	=============== ============================================
 	  mapping		spec.info means
+==	=============== ============================================
  0	hard link	link destination [file header]
  1	directory	first file's header
  2	regular file	unused, must be zero [MBZ]
@@ -123,6 +129,7 @@ the following:
  5	char device		    - " -
  6	socket		unused, MBZ
  7	fifo		unused, MBZ
+==	=============== ============================================
 
 Note that hard links are specifically marked in this filesystem, but
 they will behave as you can expect (i.e. share the inode number).
@@ -183,4 +190,5 @@ limiting factor in my eyes.
 
 
 Have fun,
+
 Janos Farkas <chexum@shadow.banki.hu>