Add release script

First step to automate release procedure.

RELEASE.md

@@ -1,12 +1,5 @@
-- Update version at:
-	- `testslide/version`
-- Commit & push the tag:
-  - `git checkout master`
-  - `git add testslide/version`
-  - `git commit -m "v$VERSION"`
-  - `git tag $VERSION`
-  - `git push`
-  - `git push master --tags`
+- Create a new release tag by running:
+	- `./release.sh $new_version`
 - https://travis-ci.com/facebookincubator/TestSlide/
 	- Check if master build is OK.
 - https://readthedocs.org/projects/testslide/

release.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+set -euxo pipefail
+
+USAGE="No release version specified. Please try again like this: $0 <release_version>"
+
+release_version=${1?$USAGE}
+
+git checkout master
+git pull
+
+echo $release_version > testslide/version
+git add testslide/version
+git commit -m "v$release_version"
+git push
+
+git tag $release_version
+git push origin $release_version