minor symfony#12468 Update filesystem readme.md to include exists method (ericduran)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#12468).

Discussion
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Update filesystem readme.md to include exists method

Read the contributing docs, I don't think I missed anything but to be fair I went through it pretty fast :-/

Commits
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16c1b66 Update filesystem readme.md to include exists method

Author: fabpot

src/Symfony/Component/Filesystem/README.md

@@ -18,6 +18,8 @@ $filesystem->touch($files, $time = null, $atime = null);
 
 $filesystem->remove($files);
 
+$filesystem->exists($files);
+
 $filesystem->chmod($files, $mode, $umask = 0000, $recursive = false);
 
 $filesystem->chown($files, $user, $recursive = false);