Add building instructions to CONTRIBUTING.md

[ericcornelissen]

Added instructions to CONTRIBUTING.md on how to build and run the site locally. As someone who has virtually no experience with Jekyll, I just wrote down what I did to get it working 😅 If there is something wrong with it, or something that could be made simpler, please feel free to suggest any changes!

[birjolaxew]

Thanks! I've been thinking of writing this, but it must've slipped my mind ^^

Are you sure you cannot simply jekyll serve, without step 1-3? IIRC, bundle exec jekyll serve fails, but jekyll by itself works fine.

[ericcornelissen]

@birjolaxew you're absolutely right that jekyll serve is all you really need 👍 That way Jekyll also doesn't generate a ton of files that are not ignored by git 😅

[birjolaxew]

👍 Going to leave this open until tomorrow to allow time for comments.

[benjaminwil]

👌