README: mention Prism for syntax highlighting

Author: simon04

README.md

@@ -87,6 +87,7 @@ Modifications made to each document include:
 * replacing all external (not scraped) URLs with their fully qualified counterpart
 * replacing all internal (scraped) URLs with their unqualified and relative counterpart
 * adding content, such as a title and link to the original document
+* ensuring correct syntax highlighting using [Prism](http://prismjs.com/)
 
 These modifications are applied via a set of filters using the [HTML::Pipeline](https://github.com/jch/html-pipeline) library. Each scraper includes filters specific to itself, one of which is tasked with figuring out the pages' metadata.