Create files needed to run Jekyll locally

By adding a Gemfile, it is possible to test the website/documentation
rendering locally.  This expects a reasonably recent version of "bundle"
to be installed, and from within the docs directory:

    bundle update
    bundle exec jekyll serve

will run a small web server on port 4000 to preview the docs.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/Gemfile b/docs/Gemfile
new file mode 100644
index 0000000..047e5e2
--- /dev/null
+++ b/docs/Gemfile
@@ -0,0 +1,27 @@
+source "https://rubygems.org"
+
+# Hello! This is where you manage which Jekyll version is used to run.
+# When you want to use a different version, change it below, save the
+# file and run `bundle install`. Run Jekyll with `bundle exec`, like so:
+#
+#     bundle exec jekyll serve
+#
+# This will help ensure the proper Jekyll version is running.
+# Happy Jekylling!
+gem "jekyll", "3.5.2"
+
+# This is the default theme for new Jekyll sites. You may change this to anything you like.
+# gem "jekyll-theme-cayman", "~> 0.1"
+
+# If you want to use GitHub Pages, remove the "gem "jekyll"" above and
+# uncomment the line below. To upgrade, run `bundle update github-pages`.
+gem "github-pages", group: :jekyll_plugins
+
+# If you have any plugins, put them here!
+# group :jekyll_plugins do
+#    gem "jekyll-feed", "~> 0.6"
+# end
+
+# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
+gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
+