Add some basic usage documentation

Some of the current input and output values
are purposely left undocumented, since they
are likely to change or be removed in the next major release.

Signed-off-by: Jonathan Schwender <[email protected]>

README.md

@@ -1,4 +1,41 @@
 # OHOS SDK
 
 This is a simple GitHub action to automatically download and install the OpenHarmony SDK,
-so you can use it in your GitHub actions workflow.
+so you can use it in your GitHub actions workflow.
+
+## Usage
+
+```yaml
+name: Main
+
+on: push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup OpenHarmony SDK
+        uses: openharmony-rs/[email protected]
+```
+
+### Options
+
+**inputs**:
+
+| Name       | Type    | Default | Description                                                                                                                |
+|------------|---------|---------|----------------------------------------------------------------------------------------------------------------------------|
+| version    | String  | 4.1     | Version of the OpenHarmony SDK (e.g. `4.0`, `4.1` or `5.0`)                                                                |
+| cache      | Boolean | true    | Uses the GitHub actions cache to cache the SDK when enabled.                                                               |
+| components | String  | all     | SDK components that should be added. `all` or semicolon seperated list of components.                                      |
+| mirror     | Boolean | true    | Download from Github Releases mirror of the SDK if possible.                                                               |
+
+**outputs**:
+
+
+| Name            | Type   | Description                                            |
+|-----------------|--------|--------------------------------------------------------|
+| api-version     | String | API Version of the SDK (e.g. `12` for OpenHarmony 5.0) |
+| sdk-version     | String | Specific SDK version (e.g. `4.1.7.5`)                  |
+| ohos_sdk_native | String | Path to the `native` directory in the OpenHarmony SDK. |