docs: add readme

Author: stigok

README.md

@@ -0,0 +1,65 @@
+# yaml-schema-validator-github-action
+
+A GitHub action that uses [Yamale][] for YAML schema validation.
+
+## Usage
+
+Configure a new workflow in your project referencing this repository.
+The following example sets up a check to validate the file *target.yaml*
+using the Yamale schema defined in *schemas/schema.yaml* within the target
+repository.
+
+- Filenames are relative to the repository root.
+- Enable strict checking by setting `strict` to a non-empty string.
+- For help with the schema definitions, see [Yamale][].
+
+```
+name: YAML schema validator
+on: [push]
+
+jobs:
+  yaml-schema-validation:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: nrkno/yaml-schema-validator-github-action@v0.0.1-pre
+      with:
+        schema: 'schemas/schema.yaml'
+        target: 'target.yaml'
+        # Uncomment to enable strict checks
+        # strict: '1'
+```
+
+## Developing
+
+Create and enable a Python virtualenv (not strictly required, but makes testing
+more robust)
+
+```
+$ python -m venv venv
+$ source venv/bin/activate
+```
+
+Install dependencies
+
+```
+$ pip install -r requirements.txt
+```
+
+Do a test-run with the provided examples
+
+```
+$ ./entrypoint.sh example/schema.yaml example/file.yaml
+```
+
+### Using Docker
+
+Build the container and reference files within the example/ folder.
+
+```
+$ docker build -t yaml-schema-validator .
+$ docker run yaml-schema-validator example/schema.yaml example/file.yaml
+```
+
+
+[Yamale]: https://github.com/23andMe/Yamale