Please add the link of the supported currencies list by ECB to the Readme.md file

[joharkhan99]

Please add the below link to the readme file. I have checked the issues on this repo and most of them were related to currency not being supported. Adding this link would greatly clarify the list of supported currencies to users and there won't be any new issues regarding to currency not supported.

https://www.ecb.europa.eu/stats/policy_and_exchange_rates/euro_reference_exchange_rates/html/index.en.html

thanks.