Update README.md

Author: redisdev

README.md

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 [![NuGet](https://img.shields.io/nuget/v/FakeApi.svg)](https://www.nuget.org/packages/FakeApi/)
 
+[![codecov](https://img.shields.io/azure-devops/coverage/redisdev/FakeApi/1.svg)](https://dev.azure.com/redisdev/FakeApi/_build/results?buildId=55&view=codecoverage-tab)
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 # What is FakeApi?
 
 FakeApi provides the ability to send HttpWebRequest and get HttpWebResponses without a server.