add some note with php calendar extension

[moghwan]

in some cases (as it happened to me) the calendar extension is disabled.

[khaled-alshamaa]

Thank you @moghwan for this notification! Well, it was a bit weird to find a PHP server setup without calendar extension, it's rare, but a good catch as well.

But instead of mentioning that in the project readme file, I prefer if you include it in the composer.json file just like what we did for the mbstring extension:

"ext-calendar": "*"

Please update your PR accordingly to merge it and welcome you on the Ar-PHP contributers my friend Marwane ;-)

[moghwan]

It's done, I reverted back the edit of README.md as well.
FYI it happened to me because of my LAMPP stack install from scratch and the default php.ini had the extension disabled; Thus an error occured as Call to undefined function ArPHP\I18N\gregoriantojd().

And thanks for the library Mr Khaled :D.

edit: original issue described