docs: add jemoji plugin

This plugin enables jekyll to display emojis; the version added matches
current on github-pages (https://pages.github.com/versions/)

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/Gemfile b/docs/Gemfile
index 7b9e0cb..fd9a462 100644
--- a/docs/Gemfile
+++ b/docs/Gemfile
@@ -20,3 +20,5 @@
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
 gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
+
+gem "jemoji", "~> 0.12.0"