ZOOKEEPER-2880 Rename README.txt to README.md - fixed packaging

(cherry picked from commit 37e277162d567b55a07d1755f0b31c32e93c01a0)

build.xml

@@ -831,7 +831,7 @@ xmlns:cs="antlib:com.puppycrawl.tools.checkstyle.ant">
 
       <copy todir="${dist.dir}">
         <fileset file="CHANGES.txt"/>
-        <fileset file="README.txt"/>
+        <fileset file="README.md"/>
         <fileset file="README_packaging.txt"/>
         <fileset file="build.xml"/>
         <fileset file="ivy.xml"/>
@@ -948,7 +948,7 @@ xmlns:cs="antlib:com.puppycrawl.tools.checkstyle.ant">
 
       <copy todir="${dist.dir}">
         <fileset file="CHANGES.txt"/>
-        <fileset file="README.txt"/>
+        <fileset file="README.md"/>
         <fileset file="README_packaging.txt"/>
         <fileset file="build.xml"/>
         <fileset file="ivy.xml"/>