Add readme and license (#5)

Author: rosik

LICENSE

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+
+The MIT License (MIT)
+
+Copyright (c) 2018 GitHub, Inc. and contributors
+
+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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+# Setup Tarantool
+
+This action will set up [Tarantool](https://www.tarantool.io) environment and **cache the packages**.
+
+- When cached, it takes \~1-2s to finish.
+- The first run takes \~40s.
+- Cache size is \~20MB.
+- Runs on `ubuntu-*` only.
+
+# Usage
+
+### Basic
+
+```yaml
+steps:
+  - uses: actions/checkout@v2
+  - uses: rosik/setup-tarantool@v1
+    with:
+      tarantool-version: '2.5'
+  - run: tarantoolctl rocks make
+```
+
+### Custom cache key
+
+By default, the action caches installed apt packages with the key:
+
+```tarantool-setup-${tarantool-version}-${ubuntu-version}```
+
+If you need to drop the cache, it's customizable:
+
+```yaml
+steps:
+  - uses: rosik/setup-tarantool@v1
+    with:
+      tarantool-version: 2.5
+      cache-key: some-other-key
+```
+
+# License
+
+The scripts and documentation in this project are released under the [MIT License](LICENSE).
+
+# Contributions
+
+Contributions are welcome! Just open an issue or send a pull request.