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README.md

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+# GitHub Action: Coverage Comment
+
+Publish diff coverage report as PR comment, and create a coverage badge to
+display on the readme.
+
+## What does it do?
+
+This action operates on an already generated `coverage.xml` (Cobertura) file as
+generated by most tools, accross languages.
+
+It has two main modes of operation:
+
+### PR mode
+
+If acting on a PR, it will analyze the XML file, and produce a comment that
+will be posted to the PR. If a comment had already previously be written,
+it will be updated. The comment contains information on the evolution
+of coverage rate attributed to this PR, as well as the rate of coverage
+for lines that this PR introduces. There's also a small analysis for each
+file in a collapsed block.
+
+See: https://github.com/ewjoachim/coverage-comment-action-example/pull/3#issuecomment-927207016
+
+# Default branch mode
+
+If acting on the repository's default branch, it will extract the coverage
+rate and create a small JSON file that will be stored on the repository's wiki.
+This file will then have a stable URL, which means you can create a
+[shields.io](https://shields.io/endpoint) badge from it.
+
+See: https://github.com/ewjoachim/coverage-comment-action-example
+
+## Usage
+
+### Setup
+
+Please ensure that the **repository wiki has been initialized** with at least a
+single page created. Once it's done, you can disable the wiki for the
+repository.
+
+### Minimal usage
+```yaml
+- name: Display coverage
+  uses: ewjoachim/coverage-comment-action@v1
+  with:
+    GITHUB_TOKEN: ${{ github.token }}
+```
+
+### Maximal usage
+```yaml
+- name: Display coverage
+  uses: ewjoachim/coverage-comment-action@v1
+  with:
+    GITHUB_TOKEN: ${{ github.token }}
+
+    # Path and filename of the coverage XML file to analyze.
+    COVERAGE_FILE: "coverage.xml"
+
+    # Whether or not a badge will be generated and stored.
+    BADGE_ENABLED: "true"
+
+    # Name of the json file containing badge informations stored in the repo wiki.
+    BADGE_FILENAME: coverage-comment-badge.json
+
+    # If the coverage percentage is above or equal to this value, the badge will be green.
+    MINIMUM_GREEN: 100
+
+    # Same with orange. Below is red.
+    MINIMUM_ORANGE: 70
+
+    # [Advanced] Specify a different template for the comments that will be written on the PR.
+    COMMENT_TEMPLATE: ""
+
+    # [Advanced] Additional args to pass to diff cover (one per line)
+    DIFF_COVER_ARGS: ""
+```
+
+### Note on the state of this action
+
+There is no automated test and the dependencies are not frozen, so it's
+possible that it fails at some point if a dependency breaks compatibility.
+If this happens, we'll fix it and put better checks in place.
+
+It's probably usable as-is, but you're welcome to offer feedback and, if you
+want, contributions.
+
+### Note on advanced options
+
+``COMMENT_TEMPLATE`` and ``DIFF_COVER_ARGS`` are somewhat experimental.
+They haven't been thouroughly tested.