Remove hard coded memory limit. Add troubleshooting doc

bin/composer

@@ -7,7 +7,6 @@ use Composer\Console\Application;
 
 error_reporting(-1);
 @ini_set('display_errors', 1);
-@ini_set('memory_limit', '512M');
 
 // run the command application
 $application = new Application();

doc/articles/troubleshooting.md

@@ -0,0 +1,25 @@
+<!--
+    tagline: Solving problems
+-->
+# Memory limit errors
+
+If composer shows memory errors on some commands:
+
+    PHP Fatal error:  Allowed memory size of XXXXXX bytes exhausted <...>
+
+The `memory_limit` ini value should be increased.
+
+Get current value:
+
+    php -r "echo ini_get('memory_limit').PHP_EOL;"
+
+
+Increase limit with `php.ini` for a `CLI SAPI` (ex. `/etc/php5/cli/php.ini` for Debian-like systems):
+
+    ; Use -1 for unlimited or define expicit value like 512M
+    memory_limit = -1
+
+Or with comand line arguments:
+
+    php -d memory_limit=-1 composer.phar <...>
+