minor #7645 Adding a new article about "Creating a Bug Reproducer" (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes #7645).

Discussion
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Adding a new article about "Creating a Bug Reproducer"

It's very common for us to tell issue reportes to create a bug reproducer. We usually tell them *"fork the Symfony Standard edition and reproduce the bug"* ... but I don't like that and I'm sure that most people don't understand what we're talking about.

I propose to add this article to explain as simple as possible what do we expect them to do, so we can link to this article in every issue of the Symfony code repo.

---

If you are thinking about merging this article with others of the "Contributing" section ... let's do that later. I want to change a lot of things here ... but let's do one thing at a time. Thanks!

Commits
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b5a61ca Adding a new article about "Creating a Bug Reproducer"

Author: xabbuh

contributing/code/index.rst

@@ -5,6 +5,7 @@ Contributing Code
     :maxdepth: 2
 
     bugs
+    reproducer
     patches
     maintenance
     core_team

contributing/code/reproducer.rst

@@ -0,0 +1,77 @@
+Creating a Bug Reproducer
+=========================
+
+The main Symfony code repository receives thousands of issues reports per year.
+Some of those issues are so obvious or easy to understand, that Symfony Core
+developers can fix them without any other information. However, other issues are
+much harder to understand because developers can't easily reproduce them in their
+computers. That's when we'll ask you to create a "bug reproducer", which is the
+minimum amount of code needed to make the bug appear when executed.
+
+Reproducing Simple Bugs
+-----------------------
+
+If you are reporting a bug related to some Symfony component used outside the
+Symfony framework, it's enough to share a small PHP script that when executed
+shows the bug::
+
+    // First, run "composer require symfony/validator"
+    // Then, execute this file:
+    <?php
+    require_once __DIR__.'/vendor/autoload.php';
+    use Symfony\Component\Validator\Constraints;
+
+    $wrongUrl = 'http://example.com/exploit.html?<script>alert(1);</script>';
+    $urlValidator = new Constraints\UrlValidator();
+    $urlConstraint = new Constraints\Url();
+
+    // The URL is wrong, so var_dump() should display an error, but it displays
+    // "null" instead because there is no context to build a validator violation
+    var_dump($urlValidator->validate($wrongUrl, $urlConstraint));
+
+Reproducing Complex Bugs
+------------------------
+
+If the bug is related to the Symfony Framework or if it's too complex to create
+a PHP script, it's better to reproduce the bug by forking the Symfony Standard
+edition. To do so:
+
+1. Go to https://github.com/symfony/symfony-standard and click on the **Fork**
+   button to make a fork of that repository or go to your already forked copy.
+2. Clone the forked repository into your computer:
+   ``git clone git://github.com/YOUR-GITHUB-USERNAME/symfony-standard.git``
+3. Browse the project and create a new branch (e.g. ``issue_23567``,
+   ``reproduce_23657``, etc.)
+4. Now you must add the minimum amount of code to reproduce the bug. This is the
+   trickiest part and it's explained a bit more later.
+5. Add, commit and push all your changes.
+6. Add a comment in your original issue report to share the URL of your forked
+   project (e.g. ``https://github.com/YOUR-GITHUB-USERNAME/symfony-standard/tree/issue_23567``)
+   and, if necessary, explain the steps to reproduce (e.g. "browse this URL",
+   "fill in this data in the form and submit it", etc.)
+
+Adding the Minimum Amount of Code Possible
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The key to create a bug reproducer is to solely focus on the feature that you
+suspect is failing. For example, imagine that you suspect that the bug is related
+to a route definition. Then, after forking the Symfony Standard Edition:
+
+1. Don't edit any of the default Symfony configuration options.
+2. Don't copy your original application code and don't use the same structure
+   of bundles, controllers, actions, etc. as in your original application.
+3. Open the default controller class of the AppBundle and add your routing
+   definition using annotations.
+4. Don't create or modify any other file.
+5. Execute the ``server:run`` command and browse the previously defined route
+   to see if the bug appears or not.
+6. If you can see the bug, you're done and you can already share the code with us.
+7. If you can't see the bug, you must keep making small changes. For example, if
+   your original route was defined using XML, forget about the previous route
+   annotation and define the route using XML instead. Or maybe your application
+   uses bundle inheritance and that's where the real bug is. Then, forget about
+   AppBundle and quickly generate a new AppParentBundle, make AppBundle inherit
+   from it and test if the route is working.
+
+In short, the idea is to keep adding small and incremental changes to the default
+Symfony Standard edition until you can reproduce the bug.