docs: add Travis build status badge

Author: nklayman

docs/README.md

@@ -21,7 +21,8 @@ footer: MIT Licensed | Copyright © 2018-present Noah Klayman
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+Build Status:
 
-AppVeyor build status: [![Build status](https://ci.appveyor.com/api/projects/status/tyrr8kemli4vfll7?svg=true)](https://ci.appveyor.com/project/nklayman/vue-cli-plugin-electron-builder)
+Windows (Appveyor): [![Build status](https://ci.appveyor.com/api/projects/status/tyrr8kemli4vfll7?svg=true)](https://ci.appveyor.com/project/nklayman/vue-cli-plugin-electron-builder), Linux (Travis): [![Build Status](https://travis-ci.org/nklayman/vue-cli-plugin-electron-builder.svg?branch=master)](https://travis-ci.org/nklayman/vue-cli-plugin-electron-builder)
 
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