README: Use `X' and `x' consistently

The text referred to `XX', when only a single `X' actually
existed, so this commit changes it to just `X'; a similar
change has been made for `xx'.

Also, `N' was used when it would have been more consistent
to use `X' again, so this commit replaces `N' with `X'.

This commit also chooses to leave `X' as the placeholder
for the current version of the source, and then makes `x'
the placeholder for any version number; fortunately nothing
really depends on this subtlety being understood.

Because `x' is already being used when discussing version
numbers, this commit changes such filename references as
`xxx.rej' to `some-file-name.rej'.

Signed-off-by: Michael Witten <mfwitten@gmail.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>