Add lib description and improve demo steps

Author: pspanja

README.md

@@ -6,9 +6,30 @@
 [![Latest stable](https://img.shields.io/packagist/v/netgen/query-translator.svg?style=flat-square)](https://packagist.org/packages/netgen/query-translator)
 [![License](https://img.shields.io/packagist/l/netgen/query-translator.svg?style=flat-square)](https://packagist.org/packages/netgen/query-translator)
 
-## How to run a demo
+Query Translator is a search query [translator](https://en.wikipedia.org/wiki/Translator_(computing))
+with [abstract syntax tree](https://en.wikipedia.org/wiki/Abstract_syntax_tree) representation.
+It takes a search query as a user input and converts it into something a specific search
+backend can understand. AST representation means that output of the parser is a hierarchical tree
+structure. It represents the syntax of the given query in an abstract way and it's easy to process
+using [tree traversal](https://en.wikipedia.org/wiki/Tree_traversal).
 
-1. Clone the repository
+Query language implementation is named [Galach](https://github.com/netgen/query-translator/tree/master/lib/Languages/Galach).
+
+### Use cases
+
+1. Common query language on top of multiple search backends
+2. Better control over options of the query language provided by the search backend
+3. Post-processing user's query input
+4. Customization of the query language
+5. ...
+
+This implementation is a [library](https://en.wikipedia.org/wiki/Library_(computing)), meaning it
+doesn't intend to solve a specific use case for query translation. Instead, it's meant as a base that
+you can use in implementing such a use case.
+
+## Run the demo
+
+1. Clone the repository and position into it
 2. Generate autoloader using composer `composer dump-autoload -o`
 3. Start the web server with demo document root `php -S localhost:8005 -t demo`
-4. Open http://localhost:8005 in your browser
+4. Open [http://localhost:8005](http://localhost:8005) in your browser