Merge pull request scala#6014 from Philippus/spec-missing-step

Add missing 'bundle install' step

Author: SethTisue

spec/README.md

@@ -18,8 +18,9 @@ We aim to track the configuration GitHub Pages use but at times differences will
 
 Travis CI builds the spec automatically after every merged pull release and publishes to http://www.scala-lang.org/files/archive/spec/2.12/.
 
-To preview locally, run `bundle exec jekyll serve -d build/spec/ -s spec/ -w --baseurl=""` (in the root of your checkout of scala/scala),
-and open http://0.0.0.0:4000/. Jekyll will rebuild as you edit the markdown, but make sure to restart it when you change `_config.yml`.
+To preview locally, run the following commands in the root of your checkout scala/scala:
+`bundle install` to install Jekyll and `bundle exec jekyll serve -d build/spec/ -s spec/ -w --baseurl=""` to start it,
+and open http://0.0.0.0:4000/ to view the spec. Jekyll will rebuild as you edit the markdown, but make sure to restart it when you change `_config.yml`.
 
 ## General Advice for editors