Add coverage badge

[renatomassaro]

@mememori I've also replaced true/false with uft8 marks, which I think makes the table clearer (less bloated). Take a look at the diff, what do you think?

Also, just to state my opinion, I think the first line of the README should be Helix. It's more standard/expected behavior when compared to other repos. But personally, I don't care 😊

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This change is 

[coveralls]

Coverage decreased (-1.1%) to 59.977% when pulling 067d88c on renatomassaro:coveralls-badge into 4a98334 on HackerExperience:master.

[coveralls]

Coverage decreased (-1.4%) to 59.688% when pulling 1075c72 on renatomassaro:coveralls-badge into 4a98334 on HackerExperience:master.

[renatomassaro]

I'll remove coveralls comments because they will flood longer PRs.
Coveralls status check will fail if total coverage drops below 50% or pr changes decrease total coverage in 10%. Once we get a better coverage we can optimize these numbers (100% coverage is usually overkill, 90% is a nice threshold)

[umamaistempo]

lgtm