docs: update README.md (#43)

Add `Getting Started` and `Examples` section

Author: vbreuss

README.md

@@ -5,4 +5,35 @@
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-This library is an extension to [FluentAssertions](https://github.com/fluentassertions/fluentassertions) for the `IFileSystem` interface from [Testably.Abstractions](https://github.com/Testably/Testably.Abstractions).
+This library is an extension to [FluentAssertions](https://github.com/fluentassertions/fluentassertions) for the [`IFileSystem` interface](https://github.com/TestableIO/System.IO.Abstractions).
+
+## Getting Started
+
+- Install `Testably.Abstractions.FluentAssertions` as nuget package.
+  ```ps
+  dotnet add package Testably.Abstractions.FluentAssertions
+  ```
+
+- Add the following `using` statement:
+  ```csharp
+  using Testably.Abstractions.FluentAssertions;
+  ```
+  This brings the extension methods in the current scope.
+
+## Examples
+
+1. Verify, that a directory "foo" exists under the current directory in the file system:
+   ```csharp
+   fileSystem.Should().HaveDirectory("foo");
+   ```
+   or
+   ```csharp
+   IDirectoryInfo directoryInfo = fileSystem.DirectoryInfo.New(".");
+   directoryInfo.Should().HaveSingleDirectory("foo");
+   ```
+
+3. Verify, that the file "foo.txt" has text content "bar":
+   ```csharp
+   fileSystem.Should().HaveFile("foo.txt")
+       .Which.HasContent("bar");
+   ```