Add changes.sh script (#1416)

The changes.sh script can be used to generate PR descriptions for badger
update PRs on dgraph.

changes.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -e
+GHORG=${GHORG:-dgraph-io}
+GHREPO=${GHREPO:-badger}
+cat <<EOF
+This description was generated using this script:
+\`\`\`sh
+`cat $0`
+\`\`\`
+Invoked as:
+
+    `echo GHORG=${GHORG} GHREPO=${GHREPO} $(basename $0) ${@:1}`
+
+EOF
+git log --oneline --reverse ${@:1} \
+  | sed -E "s/^(\S{7}\s)//g" \
+  | sed -E "s/([\s|\(| ])#([0-9]+)/\1${GHORG}\/${GHREPO}#\2/g" \