README: add build, coverage, LoC badges

[gaborigloi]

Feel free to add or remove badges from this PR.

[coveralls]

Coverage decreased (-0.01%) to 13.934% when pulling f93a876 on gaborigloi:add_badges into 7a11e2d on xapi-project:master.

[gaborigloi]

It seems there is some random noise in the coverage results ⤴. Probably our tests are a bit random?

[gaborigloi]

If this random noise keeps happening in our PRs, I will adjust the "Coverage Decrease Threshold for failure" in Coveralls accordingly, to filter out this noise.

[mseri]

Yes, there are a few test employing random generation of data. Maybe we should get some reasonably big threshold there to avoid these failures. I don't know how much, but I guess 0.5% or 1% is much more than enough

[gaborigloi]

I've set it to 0.02%, as the other PR consistently had a 0.01% decrease, and we can increase it later if it's too low. 1% seems to be too much, as 1% of 100K is 1000 untested lines of code (although this count includes non-ocaml code as well). Not sure whether the line count is important enough to show as a badge.

[mseri]

I have a PR introducing pylint for our codebase as well... however it is far from being ready