Add README for actions-expressions

actions-expressions/README.md

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+# actions-expressions
+
+`actions-expressions` is a library to parse and evaluate GitHub Actions [expressions](https://docs.github.com/actions/learn-github-actions/expressions).
+
+## Installation
+
+The [package](https://www.npmjs.com/package/@github/actions-expressions) contains TypeScript types and compiled ECMAScript modules.
+
+```bash
+npm install @github/actions-expressions
+```
+
+## Usage
+
+### Simple example
+
+```ts
+import { Parser, Lexer, Evaluator, data } from '@actions/expressions';
+
+const lexer = new Lexer("1 == 2");
+const lr = lexer.lex();
+
+const parser = new Parser(lr.tokens, [], []);
+const expr = parser.parse();
+
+const evaluator = new Evaluator(expr, new data.Dictionary());
+const result = evaluator.evaluate();
+
+console.log(result.coerceString()) // false
+```
+
+### With context data
+
+```ts
+import { Parser, Lexer, Evaluator, data } from '@actions/expressions';
+
+const lexer = new Lexer("'monalisa' == context.name");
+const lr = lexer.lex();
+
+const parser = new Parser(lr.tokens, ["context"], []);
+const expr = parser.parse();
+
+const evaluator = new Evaluator(expr, new data.Dictionary([{
+  key: "context"
+  value: new data.Dictionary([{
+    key: "name"
+    value: new data.StringData("monalisa")
+  }])
+}]));
+const result = evaluator.evaluate();
+
+console.log(result.coerceString()) // true
+```
+
+## Contributing
+
+See also [CONTRIBUTING.md](../CONTRIBUTING.md) at the root of the repository.
+
+This project is just one of multiple implementations of the GitHub Actions Expressions language. We therefore cannot accept contributions that add new language features or significantly change the behavior or existing language features. If you would like to propose a change to the language itself, please use our [Community Forum](https://github.com/community/community/discussions/categories/actions-and-packages).
+
+If you do want to contribute, please run [prettier](https://prettier.io/) to format your code and add unit tests as appropriate before submitting your PR. [./testdata](./testdata) contains test cases that all implementations should pass, please also make sure those tests are still passing.
+
+## License
+
+This project is licensed under the terms of the MIT open source license. Please refer to [MIT](../LICENSE) for the full terms.