Added build status badges

README.md

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 cpp-httplib
 ===========
 
+[![Build Status](https://travis-ci.org/yhirose/cpp-httplib.svg?branch=master)](https://travis-ci.org/yhirose/cpp-httplib)
+[![Bulid Status](https://ci.appveyor.com/api/projects/status/github/yhirose/cpp-httplib?branch=master&svg=true)](https://ci.appveyor.com/project/yhirose/cpp-httplib)
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 A C++11 header-only HTTP library.
 
 It's extremely easy to setup. Just include **httplib.h** file in your code!