README.md: Add status badges.

Add status badges for Travis, Appveyor and Codecov.

Author: roberthoenig

README.md

@@ -1,3 +1,14 @@
+| **Build status** | **Coverage** |
+|:-----------------:|:----------------:|
+| [![][travis-img]][travis-url] [![][appveyor-img]][appveyor-url] | [![][codecov-img]][codecov-url] |
+
 # FirstOrderLogic
 
 This is a Julia package for parsing, manipulating and evaluating formulas in first-order logic.
+
+[codecov-img]: https://codecov.io/gh/roberthoenig/FirstOrderLogic.jl/branch/master/graph/badge.svg
+[codecov-url]: https://codecov.io/gh/roberthoenig/FirstOrderLogic.jl
+[travis-img]: https://travis-ci.org/roberthoenig/FirstOrderLogic.jl.svg?branch=master
+[travis-url]: https://travis-ci.org/roberthoenig/FirstOrderLogic.jl
+[appveyor-img]: https://ci.appveyor.com/api/projects/status/rn8exp4696vo9co5/branch/master?svg=true
+[appveyor-url]: https://ci.appveyor.com/project/roberthoenig/firstorderlogic-jl/branch/master