Fix: document the parameters in the README (#2)

README.md

@@ -3,8 +3,35 @@
 This GitHub Actions runs flake8 over your code.
 Any warnings or errors will be annoted in the Pull Request.
 
-## Example usages
+## Usage
 
 ```
 uses: TrueBrain/actions-flake8@master
 ```
+
+### Parameter: path
+
+Indicates the path to run `flake8` in.
+This can be useful if your project is more than Python code.
+
+This parameter is optional; by default `flake8` will run on your whole repository.
+
+```
+uses: TrueBrain/actions-flake8@master
+with:
+  path: src
+```
+
+### Parameter: only_warn
+
+Only warn about problems.
+All errors and warnings are annotated in Pull Requests, but it will act like everything was fine anyway.
+(In other words, the exit code is always 0.)
+
+This parameter is optional; setting this to any value will enable it.
+
+```
+uses: TrueBrain/actions-flake8@master
+with:
+  only_warn: 1
+```