Add documentation for core package (#22)

* Add documentation

* Add example code

* Fix query

* Add example docu and changed completion trigger

* Added a test for completions

README.md

@@ -1,3 +1,5 @@
 # Langium/SQL
 
-This project is going to provide a language server for SQL dialects.
+This project provides a language server for SQL dialects.
+
+Go to the [core package](./packages/langium-sql/README.md) for better documentation.

packages/langium-sql/README.md

@@ -0,0 +1,62 @@
+# Langium/SQL
+
+This project provides a language server for SQL dialects.
+
+## Features
+
+* **Schema-driven**: Add a set of table definitions to spread out the world for your SELECT queries.
+  The table definitions can be located in a different file of the same workspace. You can keep definitions and queries separated.
+* **Code completion**: Press Ctrl + Space keys to trigger the completion directly. You will get suggestions for the current context.
+* **Syntax highlighting**: to distinguish what are keywords, identifiers, numeric literals and for a better perception of the SQL syntax.
+* **Symbol search**: Use Cmd or Ctrl + mouse click on a column name to find the definition of it or explore the places where a column is used.
+* **Fast feedback** about contextual correctness: Whether referenced columns exist or types on certain operators are matching.
+* **Super-set approach**: Any piece of any dialect that is missing can be added to the main grammar and be protected from other dialects using validations.
+* **Highly customizable**: Any behavior or aspect that is missing for your specific use case can be easily overwritten.
+
+![](docs/langium_sql.svg)
+
+## Demo
+
+We prepared a showcase on the Langium project website. You can find it [here](https://langium.org/showcase/sql/).
+
+The given schema describes an airport database. It contains tables for airports, airlines, flights, and passengers. The queries are written in the MySQL dialect.
+
+## Getting started
+
+### Installation
+
+```sh
+npm install langium-sql
+```
+
+or
+
+```sh
+yarn add langium-sql
+```
+
+## Example usage
+
+Here is a minimal example of how to use the Langium/SQL language server in your project.
+
+```ts
+import { parseHelper, createTestServices } from "../test-utils";
+import { join } from "path";
+import { MySqlDialectTypes } from "../../src/dialects/mysql/data-types";
+
+const services = createTestServices(MySqlDialectTypes);
+const parse = await parseHelper(services.Sql, join('path', 'to', 'schemas.sql'));
+const document = await parse("SELECT * FROM my_table;");
+
+console.log(document.parseResult.value);
+```
+
+See more examples in the [Langium test folder](test/).
+
+### Learn more
+
+Look at the [announcing blog post](https://www.typefox.io/blog/langium-sql) or the [Langium documentation](https://github.com/langium/langium#documentation) to get familiar with the project structure.
+
+## License
+
+[MIT](../../LICENSE)