add info about engage frontmatter

Author: forstisabella

CONTRIBUTING.md

@@ -122,3 +122,4 @@ Front matter variables have unique functions, including the following:
 - `redirect_from`: Defaults to null. Takes an array of URLs from the front matter in a file, and generates a "stub" page at each URL at build-time. Each stub file redirects to the original file. Use the path from the root of the content directory, for example `/connections/destinations/catalog/` rather than `/docs/connections/destinations/catalog/`. **Note** We are mostly using NGINX redirects for SEO purposes. Approximately quarterly, we'll collect these and add them to NGINX.
 - `seo-changefreq`: default: `weekly `. Use the values [in the sitemap spec](https://www.sitemaps.org/protocol.html#xmlTagDefinitions). - sets the `changefreq` tag in the sitemap.xml generator, which tells search crawlers how often to check back.
 - `seo-priority`: values from `1.0` to `0.1`, default: `0.5 `. Sets the `Priority` tag in the sitemap
+- `engage`: defaults to false. Hides the connection modes table and adds a note in the Destination Info box that reads "This destination is only compatible with [Twilio Engage](https://segment.com/docs/engage/)."