Add the readme and license.

LICENSE

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+Copyright 2021 Yoichiro Tanaka
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# Increment a value GitHub Actions
+
+This GitHub Actions increments a value written in a file specified by condition automatically.
+
+# How to Use
+
+If you have the `build-number.json` file below in the `foobar/baz` directory:
+
+```text
+{"buildNumber":3}
+```
+
+The example of the `increment.yaml` YAML file in the `.github/workflows` directory is the following:
+
+```yaml
+name: Increment value test
+on:
+  push:
+    branches:
+      - main
+jobs:
+  test:
+    name: Increment value test
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v2
+        with:
+          ref: ${{ github.ref }}
+      - name: Increment value
+        uses: yoichiro/gh-action-increment-value@main
+        with:
+          target_directory: 'foobar/baz'
+          target_file: 'build_number.json'
+          prefix: 'buildNumber":'
+          suffix: '}'
+          commit_message: 'Increment the build number to '
+```
+
+* target_directory - The directory path where there is the target file.
+* target_file - The target file name.
+* prefix - The prefix string to determine the start position of the target number.
+* suffix - The suffix string to determine the end position of the target number.
+* commit_message - The commit message (automatically the number appended.).