Adds documentation for the new ISSN validator

[ajgarlag]

Q	A
Doc fix?	no
New docs?	yes symfony/symfony#7756
Applies to	2.3+
Fixed tickets

Reference document for the new ISSN validator. Please, review it as my English is not very good.

[wouterj]

- Issn: ~

[ajgarlag]

@ricardclau I've added a link to the wikipedia article for ISSN and a versionadded statement.

Waiting for more feedback.

[ricardclau]

You should surround the text you want to be the link with ` and use that text at the bottom of the rst document. Otherwise, as far as I know, the link will not work as expected.

As this is easier to see than to explain with words, please check https://raw.github.com/symfony/symfony-docs/master/reference/constraints/Luhn.rst

Notice that there is a link at the bottom but when the html is generated, link appears at the text written at line 7.

There should be something like this:

bla bla bla my text in the link_.

and at the end of the document

.. _my text in the link: http://en.wikipedia.org/wiki/whatever

[ajgarlag]

Forgive me. I'm pretty new to the rst format.

[wouterj]

please add a new line after the last line to avoid bad diffs if someone adds another link in this article

[ajgarlag]

Done!

[ricardclau]

No worries! I always have to double check on rst and even with it @wouterj corrects me most of the times ;)

Thanks for adding the link!

[ajgarlag]

If you have no new comments, I will squash the commit. Any last minute comment?

[wouterj]

No, I'm 👍 for this now.

[ajgarlag]

Squashed! Thanks for your support with my first documentation contribution.

[weaverryan]

Hi Antonio!

Very nice work and tweaks on this entry - it was basically perfect! I only had very minor tweaks at sha: 5821250

Thanks for your contribution!