Update local dev docs

Author: fsimonis

README.md

@@ -2,19 +2,18 @@
 
 ## Local development
 
-The website is using [Jekyll](https://jekyllrb.com/) static website generator and [Github pages](https://pages.github.com/).
-To run and develop it locally you would need [Ruby](https://www.ruby-lang.org/en/) and [Bundler](https://bundler.io/).
-With Ruby, you can install bundler using `gem install bundler`.
-Then install [`pre-commit`](https://repology.org/project/python:pre-commit/versions) to keep your commits clean.
+First install [`pre-commit`](https://repology.org/project/python:pre-commit/versions) to keep your commits clean.
 
-After that all you need is:
+The website is using [Jekyll](https://jekyllrb.com/) static website generator and [Github pages](https://pages.github.com/).
+To run and develop it locally you need to install [`rbenv`](rbenv.org) using `apt install rbenv ruby-build`.
+Then use `rbenv init` and follow the instructions to set it up.
+With `rbenv` installed and activated in your shell:
 
 ```bash
-git clone https://github.com/precice/precice.github.io && cd precice.github.io
+git clone --recurse-submodules https://github.com/precice/precice.github.io && cd precice.github.io
 pre-commit install
+rbenv install
 bundle install
-git submodule init
-git submodule update
 bundle exec jekyll serve -l
 ```
 
@@ -33,6 +32,14 @@ Afterwards, commit and push.
 
 Do not directly edit the content of the submodules from within the website repository. This might give ugly merge conflicts.
 
+## Pull with submodules
+
+To pull changes including submodules
+
+```bash
+git pull --recurse-submodules
+```
+
 ## Build inside a Docker container
 
 Instead of building on your system (which requires some setup the first time), you can directly serve the website from a Docker container (using the community image [`jekyll/jekyll`](https://hub.docker.com/r/jekyll/jekyll)). In this directory, after you initialize and update the git submodules, run the following: