Add run on repl.it badge to README

[5ecurealf]

This pull request adds a badge to the . This will allow users to easily run this repository in their browser, without having to set up an environment. You can learn more about Repl.it here.

[echuber2]

Thanks for the PR--this is a good idea. Note that if you use Repl.it for the examples, the run button may not work if you just set the run button to run, since each example needs a combination of make to build and then an individual binary invocation to run the executable (sometimes several different executables, depending on the example).

However, maybe we could leave this open for now or make an issue about it so people can see how to do it for themselves? I'm not sure we can officially make an endorsement for Repl.it in the instructions, although it's a cool service. It's fine if people run the examples on a public compiler site like that, but it's important that they set up a private workspace when they do the programming projects for a grade, in accordance with our school's academic integrity policies. To avoid causing confusion with too many different services [on top of the multiple OS install options], we tried to find one online service that would let students create a private workspace at the essentially free service level, and we unified a set of instructions around that. But, we may be able to simplify and improve the setup options in the future when the course is revised.