Add contributing guide.

Because I keep forgetting the command to host the site myself.

Author: HexDecimal

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+# Contributing
+
+## Testing this site locally
+
+For more advanced changes you can test this site by hosting it locally with following the instructions:
+https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/testing-your-github-pages-site-locally-with-jekyll
+
+- Install Ruby
+- Install Bundler
+- Run: `bundle install`
+- Run: `bundle exec jekyll serve`