Add missing extension requirements

[MindaugasJackunas]

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[cmfcmf]

Hi @MindaugasJackunas, thank you for this PR!
Could you elaborate on the differences between xml, libxml and simplexml? Also could you remove the curl requirement, because it is not strictly required to use this library. See: https://github.com/cmfcmf/OpenWeatherMap-PHP-Api/blob/92edee8c7d31913a20d0775baa1bb13782cd54e0/Cmfcmf/OpenWeatherMap/Fetcher/FileGetContentsFetcher.php

[MindaugasJackunas]

Hi @MindaugasJackunas, thank you for this PR!
Could you elaborate on the differences between xml, libxml and simplexml? Also could you remove the curl requirement, because it is not strictly required to use this library. See: https://github.com/cmfcmf/OpenWeatherMap-PHP-Api/blob/92edee8c7d31913a20d0775baa1bb13782cd54e0/Cmfcmf/OpenWeatherMap/Fetcher/FileGetContentsFetcher.php

I have moved ext-curl requirement to suggestion section.

Removed ext-xml requirement (has been added by mistake).

Places where ext-libxml and ext-simplexml are used:

• libxml - https://github.com/cmfcmf/OpenWeatherMap-PHP-Api/blob/master/Cmfcmf/OpenWeatherMap.php#L695
• simplexml - https://github.com/cmfcmf/OpenWeatherMap-PHP-Api/blob/master/Cmfcmf/OpenWeatherMap.php#L698

[cmfcmf]

Thank you very much!