minor #7792 Requirements: Rewrite note about PHP 5.3.16 (alexislefebvre)

This PR was merged into the 2.7 branch.

Discussion
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Requirements: Rewrite note about PHP 5.3.16

The `README.md` file was rewritten in this commit:
symfony/symfony@c7d30ca#diff-04c6e90faac2675aa89e2176d2eec7d8L19

So the link in _For more information see the Requirements section of the README_ was outdated because the _Requirements_ section was removed.

I removed the link and added an explanation that was present in the `README.md` file (see commit above).

Commits
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5bf065f Requirements: Rewrite note about PHP 5.3.16

Author: xabbuh

reference/requirements.rst

@@ -28,8 +28,8 @@ Required
 
 .. caution::
 
-    Be aware that Symfony has some known limitations when using PHP 5.3.16.
-    For more information see the `Requirements section of the README`_.
+    Be aware that PHP 5.3.16 is not suitable to run Symfony,
+    because of a `major bug in the Reflection subsystem`_.
 
 Optional
 --------
@@ -59,3 +59,4 @@ to use.
 .. _`Requirements section of the README`: https://github.com/symfony/symfony/blob/2.7/README.md#requirements
 .. _`JSON extension`: https://php.net/manual/book.json.php
 .. _`ctype extension`: https://php.net/manual/book.ctype.php
+.. _`major bug in the Reflection subsystem`: https://bugs.php.net/bug.php?id=62715