Add ability to write resolved version of SDK into the output variable (#324)

README.md

@@ -141,6 +141,54 @@ steps:
 ```
 > **Note**: It's the only way to push a package to nuget.org feed for macOS/Linux machines due to API key config store limitations.
 
+# Outputs and environment variables
+
+## Outputs
+
+### `dotnet-version`
+
+Using the **dotnet-version** output it's possible to get the installed by the action .NET SDK version. 
+
+**Single version installation**
+
+In case of a single version installation, the `dotnet-version` output contains the version that is installed by the action.
+
+```yaml
+    - uses: actions/setup-dotnet@v3
+      id: cp310
+      with:
+        dotnet-version: 3.1.422
+    - run: echo '${{ steps.cp310.outputs.dotnet-version }}' # outputs 3.1.422
+```
+
+**Multiple version installation**
+
+In case of a multiple version installation, the `dotnet-version` output contains the latest version that is installed by the action.
+
+```yaml
+    - uses: actions/setup-dotnet@v3
+      id: cp310
+      with:
+        dotnet-version: | 
+          3.1.422
+          5.0.408
+    - run: echo '${{ steps.cp310.outputs.dotnet-version }}' # outputs 5.0.408
+```
+**Installation from global.json**
+
+When the `dotnet-version` input is used along with the `global-json-file` input, the `dotnet-version` output contains the version resolved from the `global.json`.
+
+```yaml
+    - uses: actions/setup-dotnet@v3
+      id: cp310
+      with:
+        dotnet-version: | 
+          3.1.422
+          5.0.408
+        global-json-file: "./global.json" # contains version 2.2.207
+    - run: echo '${{ steps.cp310.outputs.dotnet-version }}' # outputs 2.2.207
+```
+
 ## Environment variables
 
 Some environment variables may be necessary for your particular case or to improve logging. Some examples are listed below, but the full list with complete details can be found here: https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-environment-variables