docs: add README.md (#10)

Reviewed-on: https://git.vdx.hu/voidcontext/woodpecker-plugin-nix-attic/pulls/10
Co-authored-by: Gabor Pihaj <gabor.pihaj@gmail.com>
Co-committed-by: Gabor Pihaj <gabor.pihaj@gmail.com>

README.md

@@ -0,0 +1,52 @@
+# Woodpecker plugin: nix-attic
+
+This woodpecker plugin can be configured to use a private binary cache, and it also has 
+[attic-client](ihttps://github.com/zhaofengli/attic) pre-installed.
+
+## Example configuration
+
+```yaml
+steps:
+  check:
+    image: git.vdx.hu/voidcontext/woodpecker-plugin-nix-attic:0.1.0
+    settings:
+      binary_cache: https://some-binary-cache.example.com
+      binary_cache_public_key: some-binary-cache.example.com:some-public-key
+      binary_cache_token:
+        from_secret: binary_cache_access_token
+      script: |
+       nix flake check
+       nix build
+       attic login default $PLUGIN_BINARY_CACHE_TOKEN
+       attic push some-cache $(nix path-info .#default)
+```
+
+## How to build the image locally?
+
+### On Linux
+
+```bash
+$ nix build
+$ docker load -i result
+```
+
+### On macOS
+
+The easiest way to build the image is using a previously built version of the image:
+
+```bash
+$ echo << EOF >> cmd.txt
+nix build
+nix-env -iA nixpkgs.docker-client
+docker load -i result
+EOF
+$ docker run -it                                                                   \
+  -e PLUGIN_BINARY_CACHE=https://some-binary-cache.example.com                     \
+  -e PLUGIN_BINARY_CACHE_PUBLIC_KEY=some-binary-cache.example.com:some-public-keyi \
+  -e PLUGIN_BINARY_CACHE_TOKEN=$ACCESS_TOKEN                                       \
+  -e PLUGIN_SCRIPT="$(cat cmd.txt)"                                                \
+  -v $(pwd):/opt/plugin                                                            \
+  -w /opt/plugin                                                                   \
+  -v /var/run/docker.sock:/var/run/docker.sock                                     \
+  git.vdx.hu/voidcontext/woodpecker-plugin-nix-attic:0.1.0
+```