LUCENE-9448: Move README.txt to README.md; We no lonnger have txt format README on the master.

lucene/luke/build.gradle

@@ -105,7 +105,7 @@ task standaloneAssemble(type: Sync) {
   from configurations.runtimeClasspath
 
   from(file("src/distribution"), {
-    filesMatching("README.txt", {
+    filesMatching("README.md", {
       filteringCharset = 'UTF-8'
       filter(ExpandProperties, project: antHelper)
     })

lucene/luke/src/distribution/README.md

@@ -1,6 +1,8 @@
+# Luke
+
 This is Luke, Apache Lucene low-level index inspection and repair utility.
 
 Luke requires Java ${required.java.version}. You can start it with:
 java -jar ${luke.cmd}
 
-Happy index hacking!
+Happy index hacking!