Format the documentation to make default input values easy to find

Previously, the information on whether an input had a default value -- and if so, what -- was incorporated into the input description.
This meant you had to read the entire description to find this information.

When using other actions, I have regularly had difficulty determining which inputs are required and default input values.
I would like for the users of this action to not have that experience.

Author: per1234

README.md

@@ -6,11 +6,15 @@ This action comments on the pull request with a report on the resulting change i
 
 ### `size-deltas-reports-artifact-name`
 
-Name of the workflow artifact that contains the memory usage data, as specified to the [`actions/upload-artifact`](https://github.com/actions/upload-artifact) action via its `name` input. Default "size-deltas-reports".
+Name of the workflow artifact that contains the memory usage data, as specified to the [`actions/upload-artifact`](https://github.com/actions/upload-artifact) action via its `name` input.
+
+**Default**: `"size-deltas-reports"`
 
 ### `github-token`
 
-GitHub access token used to comment the memory usage comparison results to the PR thread. Default [`GITHUB_TOKEN`](https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token).
+GitHub access token used to comment the memory usage comparison results to the PR thread.
+
+**Default**: [`GITHUB_TOKEN`](https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token).
 
 ## Example usage