contrib: rename initrd READMEs to README.md

It's an unhelpful naming scheme and one that breaks GitHub autoreadme.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13017

contrib/dracut/Makefile.am

@@ -3,4 +3,4 @@ include $(top_srcdir)/config/Shellcheck.am
 SUBDIRS = 02zfsexpandknowledge 90zfs
 SHELLCHECKDIRS = $(SUBDIRS)
 
-EXTRA_DIST = README.dracut.markdown
+EXTRA_DIST = README.md

contrib/initramfs/Makefile.am

@@ -8,5 +8,4 @@ dist_initrd_SCRIPTS = \
 SUBDIRS = conf.d conf-hooks.d hooks scripts
 SHELLCHECKDIRS = hooks scripts
 
-EXTRA_DIST = \
-	README.initramfs.markdown
+EXTRA_DIST = README.md