minor symfony#20295 Define a GitHub issue template to avoid support questions (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes symfony#20295).

Discussion
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Define a GitHub issue template to avoid support questions

We're getting a lot of issues that are in fact support questions. Should we give a try at defining a GitHub Issue template to try to minimize this problem?

The text contents are temporary. This is how they'd look:

![issue_template](https://cloud.githubusercontent.com/assets/73419/19683472/9be02128-9ab2-11e6-90a7-987ddcb49d97.png)

Commits
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eeaca5c Define a GitHub issue template to avoid support questions

Author: fabpot

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,13 @@
+| Q                | A
+| ---------------- | -----
+| Bug report?      | yes/no
+| Feature request? | yes/no
+| BC Break report? | yes/no
+| RFC?             | yes/no
+| Symfony version  | x.y.z
+
+<!--
+- Please fill in this template according to your issue.
+- For support request or how-tos, visit https://symfony.com/support
+- Otherwise, replace this comment by the description of your issue.
+-->