update readme

Signed-off-by: Bohan Chen <[email protected]>

README.md

@@ -2,3 +2,60 @@
 
 Used for testing resource (and resource type) actions without depending on the
 outside world.
+
+All source fields and parameters are optional.
+
+## Source Configuration
+
+* `mirror_self`: For use on `task.image_resource`, returns itself as an image that tasks can run on.
+
+* `initial_version`: Initial version that the resource should emit. Defaults to `mock`.
+
+* `no_initial_version`: Disable initial version, useful for testing resource triggers.
+
+* `force_version`: Always emit a version regardless of any param during `check`.
+
+* `create_files`: Example:
+  ```yaml
+  create_files:
+    file1.yml: |
+      foo: bar
+    file2.sh |
+      #!/bin/sh
+      echo "hello world"
+  ```
+
+* `check_delay`: Amount of time to sleep before returning from the `check`. Uses Go duration format.
+
+* `check_failure`: Force every check to return this error message.
+
+* `metadata`: List of name-value pairs to return as metadata on every get and put. Example:
+  ```yaml
+  metadata:
+  - name: foo
+    value: bar
+  - name: baz
+    value: qux
+  ```
+
+* `log`: Print a message on every action.
+
+## Behavior
+
+### `check`:
+
+If `force_version` is set, then only that version will ever be emitted, otherwise any version passed into the `check` is returned. This way you can generate a new version `foo` by running `fly check-resource --resource pipeline/resource --from version:foo`.
+
+### `in`:
+
+Since the mock resource is included as a base resource type, it can be used as the image for a task without requiring internet connection (unlike registry-image resource). It can also be used to create files without requiring external state.
+
+* `mirror_self_via_params`: Same as configuring `mirror_self` in source when set to true. Default false.
+
+* `create_files_via_params`: Similar to `create_files` in source; merged in so that additional (or replaced) files can be specified.
+
+### `out`:
+
+* `version`: Version to create.
+
+* `print_env`: Print all environment variables to stdout when set to true. Default false.