Fix syntax highlighting issues with Chroma

[salvalcantara]

Minor changes along the lines of https://themes.gohugo.io/beautifulhugo/ for supporting both highlight.js (theme-default) and chroma (hugo-native) highlighting solutions:

• To use highlightling.js, use these settings within config.toml:

pygmentsCodeFences = false
pygmentsUseClasses = false

[params]
highlightjs = true

• To rely on Chroma instead:

pygmentsCodeFences = true
pygmentsUseClasses = true

[params]
highlightjs = false

Using the new server side syntax highilighting provided by Chroma requires generating an additional CSS file. For example, for styling according to the trac theme, one would run:

hugo gen chromastyles --style=trac > static/css/syntax.css

• The main.css file has been changed to properly display the horizontal scrollbar in code fences when using Chroma for syntax highlighting.
• The README has been updated to point to the contributing guidelines.

[salvalcantara]

Required for displaying the horizontal scrollbars in code snippets that exceed the maximum width. The change is necessary only for the Chroma case, but it does not break anything in the highlighting.js case.