Added instruction to install into vendor directory.

.bundle/config

@@ -0,0 +1,2 @@
+---
+BUNDLE_PATH: "vendor/bundle"

README.md

@@ -8,7 +8,8 @@ To run this locally
 * [Fork this](https://github.com/jhipster/jhipster.github.io/fork) repo and clone to your file system
 * [Install Jekyll](https://help.github.com/articles/setting-up-your-github-pages-site-locally-with-jekyll/)
 * Run `bundle install` if you are running it for the first time.
-** If you install it into a vendor directory, do not forget to add that directory in the exclusions into _config.yml!
+ * If you want to avoid installing to system directories, install to vendor directory instead: `bundle install --path vendor/bundle`
+ * On macOS, if you have trouble installing `nokogiri`, try: `bundle config build.nokogiri --use-system-libraries=true --with-xml2-include="$(xcrun --show-sdk-path)"/usr/include/libxml2`
 * Run `bundle exec jekyll serve` in the cloned repo folder
 * You will be able to access the site at http://localhost:4000