Add in Arduino Library Manager

[ayushsharma82]

Hi,
Please create a release in github so that arduino library manager can pickup the library in it's Database. It's essential for setting up Travis CI for repositories where ESPAsyncWebServer is a dependency. In Travis CI dependencies are installed by calling arduino library manager shell script.

Reference: https://github.com/arduino/Arduino/blob/master/build/shared/manpage.adoc

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