Add instructions on how to build locally

In the README instructions should be added on how to build the website locally. When I try to do this, I get the following error:

➜  fdroid-website git:(master) jekyll server
Configuration file: /[...]/fdroid-website/_config.yml
            Source: /[...]/fdroid-website
       Destination: /[...]/fdroid-website/_site
      Generating... 
  Liquid Exception: Syntax Error in 'for loop' - Valid syntax: for [item] in [collection] in _includes/header.html, included in _layouts/default.html
jekyll 2.2.0 | Error:  Syntax Error in 'for loop' - Valid syntax: for [item] in [collection]

I guess that's because I did not build anything with JS before. If the data is a requirement, we should add fake data that gets overridden by the CI to only have to use Jekyll.