Add `CI Build` and `codecov` badges

[nedjulius]

Adds the last build and code coverage badges to the README.md file.

Requires #47 to be merged first

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.64%. Comparing base (687378d) to head (c0f35b6).

:exclamation: Current head c0f35b6 differs from pull request most recent head 269e25f. Consider uploading reports for the commit 269e25f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #48 +/- ## ======================================= Coverage 95.64% 95.64% ======================================= Files 11 11 Lines 390 390 ======================================= Hits 373 373 Misses 17 17 ```

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