Mention the availability of DNS plugin snaps in our docs under certbot/docs. (#8176)

Part of #8142.

* Mention that DNS plugins are available as snaps

* Mention snaps in guide to writing third-party plugins

certbot/docs/contributing.rst

@@ -340,7 +340,10 @@ only work for users who have Certbot installed from OS packages or via
 pip. Users who run `certbot-auto` are currently unable to use third-party
 plugins. It's technically possible to install third-party plugins into
 the virtualenv used by `certbot-auto`, but they will be wiped away when
-`certbot-auto` upgrades.
+`certbot-auto` upgrades. If you'd like your plugin to be used alongside
+the Certbot snap, you will also have to publish your plugin as a snap.
+Certbot's DNS plugins and the README file in ``tools/snap/`` provide a
+starting reference for how to do this.
 
 .. _`setuptools entry points`:
     http://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points

certbot/docs/using.rst

@@ -180,9 +180,9 @@ Certbot's DNS plugins.
 
 These plugins are not included in a default Certbot installation and must be
 installed separately. While the DNS plugins cannot currently be used with
-``certbot-auto``, they are available in many OS package managers and as Docker
-images. Visit https://certbot.eff.org to learn the best way to use the DNS
-plugins on your system.
+``certbot-auto``, they are available in many OS package managers, as Docker
+images, and as snaps. Visit https://certbot.eff.org to learn the best way to use
+the DNS plugins on your system.
 
 Once installed, you can find documentation on how to use each plugin at: