Update readme with a bit more info on local development

README.md

@@ -2,8 +2,24 @@ Kamon.io
 ========
 Sources for the Kamon website.
 
-Run local server
+
+Development
 ----------------
+
+For installing requirements, read the [Jekyll Docs](https://jekyllrb.com/docs/).
+
+To start the local server, run:
+
+```
+$ bundle install
+$ bundle exec jekyll serve --livereload
+```
+
+- by default, local server will be listening on port `4000`.
+- `bundle exec` restricts the Ruby environment to only use gems set in the project's `Gemfile`.
+
+For updating gem versions, run:
+
 ```
-$ jekyll serve --livereload
+$ bundle update
 ```