docs: add asdf and GitHub Actions installations (#560)

## What does this change?

Add asdf and GitHub Actions installations on README.

-
[FlutterGen/asdf-fluttergen](https://github.com/FlutterGen/asdf-fluttergen)
-
[FlutterGen/setup-fluttergen](https://github.com/FlutterGen/setup-fluttergen)

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [x] This change requires a documentation update

## Checklist:

Before submitting your PR, there are a few things you can do to make
sure it goes smoothly:

- [x] Make sure to open a GitHub issue as a bug/feature request before
writing your code! That way we can discuss the change, evaluate designs,
and agree on the general idea
  - [x] Ensure the tests (`melos run test`)
- [x] Ensure the analyzer and formatter pass (`melos run format` to
automatically apply formatting)
- [x] Appropriate docs were updated (if necessary)

README.md

@@ -61,6 +61,23 @@ Works with macOS and Linux.
 brew install FlutterGen/tap/fluttergen
 ```
 
+### asdf
+
+Works with macOS and Linux.
+asdf-fluttergen is compatible with [mise](https://mise.jdx.dev/).
+
+```sh
+# add plugin
+asdf plugin add fluttergen
+# or
+asdf plugin add fluttergen https://github.com/FlutterGen/asdf-fluttergen.git
+
+# install fluttergen
+asdf install fluttergen latest
+```
+
+See also: [FlutterGen/asdf-fluttergen](https://github.com/FlutterGen/asdf-fluttergen)
+
 ### Pub Global
 
 Works with macOS, Linux and Windows.
@@ -93,6 +110,18 @@ flutter pub get
 dart run build_runner build
 ```
 
+### GitHub Actions
+
+Works with macOS and Linux.
+
+```yaml
+- uses: FlutterGen/setup-fluttergen@v1
+  with:
+    version: ${{ fluttergen_version }}
+```
+
+See also: [FlutterGen/setup-fluttergen](https://github.com/FlutterGen/setup-fluttergen)
+
 ## Usage
 
 Run `fluttergen` after the configuration [`pubspec.yaml`](https://dart.dev/tools/pub/pubspec).