Add instructions for running jekyll locally

This commit adds instruction about running jekyll locally.

The motivation for this is that it can be useful to be able to check
the blog post formatting locally while writing them.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

Author: danbev

README.adoc

@@ -1,9 +1,39 @@
 
 Source repo for https://trustification.io[trustification.io]
 
-To add a post:
+== Add a post
 
       . Add a markdown file beneath link:./_posts/[_posts] named according to this format: `2022-10-20-some-name.md`
       . Commit your changes and push to `gh-pages` branch to publish
 
 That's it.
+
+== Running locally
+
+Make sure that you have `Ruby` install which can be installed using
+link:https://rvm.io[rvm]:
+[shell]
+----
+$ rvm install 3.0.0
+$ rvm --default use 3.0.0
+----
+
+Install `bundler` and `jekyll`:
+[shell]
+----
+$ gem install jekyll bundler
+----
+
+Install the gems required (from Gemfile):
+[shell]
+----
+$ bundle install
+----
+
+Run jekyll:
+[shell]
+----
+$ bundle exec jekyll serve
+----
+
+You should now be able to open http://127.0.0.1:4000/ in a browser.