Add C2FO/vfs to Files section. (#2396)

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- github.com repo: https://github.com/C2FO/vfs
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- coverage service link: [![codecov](https://codecov.io/gh/c2fo/vfs/branch/master/graph/badge.svg)](https://codecov.io/gh/c2fo/vfs)


Very good coverage

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README.md

@@ -602,6 +602,7 @@ Please take a quick gander at the [contribution guidelines](https://github.com/a
 * [pdfcpu](https://github.com/hhrutter/pdfcpu) - PDF processor.
 * [skywalker](https://github.com/dixonwille/skywalker) - Package to allow one to concurrently go through a filesystem with ease.
 * [tarfs](https://github.com/posener/tarfs) - Implementation of the [`FileSystem` interface](https://godoc.org/github.com/kr/fs#FileSystem) for tar files.
+* [vfs](https://github.com/C2FO/vfs) - A pluggable, extensible, and opinionated set of filesystem functionality for Go across a number of filesystem types such as os, S3, and GCS.
 
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