Improve the README

The README needed a little love. Primarily, I wanted to avoid
recommending `sudo gem install` as that's widely considered not to be a
best practice. Using a version of Ruby installed with rbenv is a better
way to avoid permission errors.

 * Recommend using rbenv to install a modern version of Ruby.
 * Avoid recommending `sudo` to install gems.
 * `bundle install --path ...` is deprecated, and isn't really necessary
   anyway. A simple `bundle install` is fine.
 * Clarify update instructions.

Author: mkasberg

README.md

@@ -1,6 +1,6 @@
 # Open Source Catholic
 
-This Jekyll project powers https://www.opensourcecatholic.com/
+This Jekyll project powers <https://www.opensourcecatholic.com/>.
 
 Open Source Catholic's mission is to help Catholic developers and organizations involved in software and web development find effective solutions for spreading the Gospel of Jesus Christ.
 
@@ -14,11 +14,15 @@ You can contribute content to Open Source Catholic following the guide in CONTRI
 
 ## Local Development
 
+Building the site locally requires Ruby. We recommend installing Ruby using [rbenv](https://github.com/rbenv/rbenv). You should install and use a version of ruby matching the one specified in [.ruby-version](.ruby-version).
+
 To get started building the site locally, run the following commands in the project directory:
 
-  1. `gem install jekyll bundler` (use `sudo` if you get a permission error)
-  2. `bundle install --path vendor/bundle`
+  1. `gem install jekyll bundler`
+  2. `bundle install`
   3. `bundle exec jekyll serve`
-  4. Browse to [http://localhost:4000/](http://localhost:4000/)
+  4. Open [http://localhost:4000/](http://localhost:4000/) in a web browser
+
+### Updating Gems
 
-To update the gems in `Gemfile.lock` (if they are out of date), run `bundle update`.
+To update the gems in `Gemfile.lock` (if they are out of date), run `bundle update` and commit the new `Gemfile.lock`.