add README

README.md

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+# Git Merge Diff
+
+This action generate git merge diff between tags.
+
+# Usage
+
+```yaml
+- uses: actions/checkout@v2
+- run: git fetch --prune --unshallow
+- name: Diff
+    id: diff
+    uses: yanamura/git-merge-diff@v1
+```
+
+> Note: don't forget to fetch.(actions/checkout only fetch depth=0)
+
+## get output(merge diff)
+
+```
+steps.<step id>.outputs.diff
+```
+
+# Senario
+
+```yaml
+- uses: actions/checkout@v2
+- run: git fetch --prune --unshallow
+- name: Diff
+    id: diff
+    uses: yanamura/git-merge-diff@v1
+- name: Create Release
+    uses: actions/create-release@v1
+    env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    with:
+        tag_name: ${{ github.ref }}
+        release_name: Release ${{ github.ref }}
+        body: |
+            ${{ steps.diff.outputs.diff }}
+        draft: false
+        prerelease: false
+```