Add install instructions with Windows DLLs

Author: nikic

README.md

@@ -2,6 +2,24 @@ This extension exposes the abstract syntax tree generated by PHP 7.
 
 This extension is experimental and the representation of the syntax tree is not final.
 
+Installation
+------------
+
+**On Windows**: Download a [prebuilt Windows DLL](http://windows.php.net/downloads/pecl/snaps/ast/)
+and move it into the `ext/` directory of your PHP instalation. Furthermore add
+`extension=php_ast.dll` to your `php.ini` file.
+
+**On Unix**: Compile and install the extension as follows.
+
+```sh
+phpize
+./configure
+make
+sudo make install
+```
+
+Additionally add `extension=ast.so` to your `php.ini` file.
+
 API overview
 ------------
 
@@ -21,12 +39,12 @@ Usage
 
 Code can be parsed using either `ast\parse_code()`, which accepts a code string, or
 `ast\parse_file()`, which accepts a file path. Additionally both functions require a `$version`
-argument to ensure forward-compatibility. The current version is 15.
+argument to ensure forward-compatibility. The current version is 20.
 
 ```php
-$ast = ast\parse_code('<?php ...', $version=15);
+$ast = ast\parse_code('<?php ...', $version=20);
 // or
-$ast = ast\parse_file('file.php', $version=15);
+$ast = ast\parse_file('file.php', $version=20);
 ```
 
 The abstract syntax tree returned by these functions consists of `ast\Node` objects.