minor symfony#28575 [WebProfilerBundle] added a note in the README (fabpot)

This PR was merged into the 2.8 branch.

Discussion
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[WebProfilerBundle] added a note in the README

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets |
| License       | MIT
| Doc PR        |

<!--
Write a short README entry for your feature/bugfix here (replace this comment block.)
This will help people understand your PR and can be used as a start of the Doc PR.
Additionally:
 - Bug fixes must be submitted against the lowest branch where they apply
   (lowest branches are regularly merged to upper ones so they get the fixes too).
 - Features and deprecations must be submitted against the master branch.
-->

Commits
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310870a [WebProfilerBundle] added a note in the README

Author: fabpot

src/Symfony/Bundle/WebProfilerBundle/README.md

@@ -1,6 +1,12 @@
 WebProfilerBundle
 =================
 
+The Web profiler bundle is a **development tool** that gives detailed
+information about the execution of any request.
+
+**Never** enable it on production servers as it will lead to major security
+vulnerabilities in your project.
+
 Resources
 ---------