Update README with fs.FS provider

Author: mvrahden

README.md

@@ -606,6 +606,7 @@ Writing Providers and Parsers are easy. See the bundled implementations in the `
 | Package             | Provider                                                      | Description                                                                                                                                                                           |
 | ------------------- | ------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | providers/file      | `file.Provider(filepath string)`                              | Reads a file and returns the raw bytes to be parsed.                                                                                                                                  |
+| providers/fs      | `fs.Provider(f fs.FS, filepath string)`                              | (**Experimental**) Reads a file from fs.FS and returns the raw bytes to be parsed. The provider requires `go v1.16` or higher.                                            |
 | providers/basicflag | `basicflag.Provider(f *flag.FlagSet, delim string)`           | Takes an stdlib `flag.FlagSet`                                                                                                                                                        |
 | providers/posflag   | `posflag.Provider(f *pflag.FlagSet, delim string)`            | Takes an `spft3/pflag.FlagSet` (advanced POSIX compatible flags with multiple types) and provides a nested config map based on delim.                                                 |
 | providers/env       | `env.Provider(prefix, delim string, f func(s string) string)` | Takes an optional prefix to filter env variables by, an optional function that takes and returns a string to transform env variables, and returns a nested config map based on delim. |