Enable php_openssl.dll in php.ini

it specifies to enable the extension php_openssl.dll in php.ini. When php_openssl.gll is disabled, the command show an error in the function file_get_contents but not specify that is necesary to enable php.ini.

doc/00-intro.md

@@ -109,6 +109,8 @@ composer.phar:
     C:\Users\username>cd C:\bin
     C:\bin>php -r "eval('?>'.file_get_contents('https://getcomposer.org/installer'));"
     
+> **Note:** If the above fails due to file_get_contents, enable php_openssl.dll in php.ini
+
 Create a new `.bat` file alongside composer:
 
     C:\bin>echo @php "%~dp0composer.phar" %*>composer.bat