docs: add custom parsers to README

Added paragraph about custom parsers to README with links to relevant code and documentation.

Author: ftrain

README.md

@@ -13,6 +13,8 @@ The Mercury Parser extracts the bits that humans care about from any URL you giv
 
 Mercury Parser powers the [Mercury AMP Converter](https://mercury.postlight.com/amp-converter/) and [Mercury Reader](https://mercury.postlight.com/reader/), a Chrome extension that removes ads and distractions, leaving only text and images for a beautiful reading view on any site.
 
+Mercury Parser allows you to easily create custom parsers using simple JavaScript and CSS selectors. This allows you to proactively manage parsing and migration edge cases. There are [many examples available](https://github.com/postlight/mercury-parser/tree/master/src/extractors/custom) along with [documentation](https://github.com/postlight/mercury-parser/blob/master/src/extractors/custom/README.md).
+
 ## How? Like this.
 
 ### Installation