Hi everyone! As my previous PR #1647, this one also comes after my experiences when setting up the project.

While doing that, I ran into an issue when with the sass command. The brief summary is:

1. It askef for another dependency that I had to install with gem install --version '~> 0.8.8' rb-inotify;
2. This command raised an error because I didn't have ruby development headers installed;
3. I had to run sudo apt install ruby-dev to fix that;
4. Only after that I was able to install the missing dependency and run sass as expected;

To help people on advance, I added a small note in the installing instruction so people don't forget to install ruby's dev headers. Also, now rb-inotify is listed in Gemfile.

I'm also proposing 2 other things with this PR with a new Makefile:

1. A make sass command to avoid people from remembering how to run sass. The current command can be very complicated to remember and this shortcut can help with that;
2. A make run command to allow people to, if they want to, run all the development commands using a single terminal window;

Before this work, I talked briefly with @ewdurbin about moving from Ruby's gems to Node.js' npm to handle the .scss files. On second thought, I realized the project would still with external dependencies to perform the same task, but from different technologies, and it didn't sound like a huge benefit IMHO. It would be nice to have second opinions on this as well and, maybe, having a new issue to address this topic.