createRelease.sh: changelog editing failed for multiline messages

sed does not accept multiline strings in its replacement text.
Let's replace newlines with literal "\n" in modify_files()

bin/createRelease.sh

@@ -65,7 +65,16 @@ function check_api_token {
 
 function modify_files {
   # Add changelog text to first line of CHANGELOG.md
-  sed -i "1s/^/${changelogText}\n/" CHANGELOG.md
+
+  msg=""
+  # source: https://unix.stackexchange.com/questions/9784/how-can-i-read-line-by-line-from-a-variable-in-bash#9789
+  while IFS= read -r line
+  do
+    # replace newlines with literal "\n" for using with sed
+    msg+="$line\n"
+  done < <(printf '%s\n' "${changelogText}")
+
+  sed -i "1s/^/${msg}\n/" CHANGELOG.md
   [[ $? != 0 ]] && echo "Aborting: Error modifying CHANGELOG.md" && exit 1
 
   # Replace version number of etherpad in package.json