Describe drift front-matter option in CONTRIBUTING.md

Author: jseldess

CONTRIBUTING.md

@@ -108,6 +108,7 @@ Field | Description | Default
 `twitter` | Adds code required to track the page as part of a Twitter campaign | `false`
 `no_sidebar` | If `true`, removes the sidebar from a page. See [Sidebar](#sidebar) for more details. | Nothing
 `block_search` | If `true`, adds meta tags to the header that excludes the page from search indexing/caching. | Nothing
+`drift` | Set this to `true` if a Drift survey is active on the page. This excludes back-to-top and help buttons from the page, which would otherwise conflict visually with the Drift interface.
 
 #### Page TOC