Update README.md

[Rakhi-2002]

added new link in Sites for preferred programming language, C++

Summary of your changes

• I have added a new link in Sites for preferred programming language( C++), CPP Reference, it contains all information about C++ language such as syntaxes, STL, Containers & other information.

Description

Checklist

• My change follows the Contributing Guidelines
• I have added only one new link to the list.
• I have checked that the link that I added does NOT exist in the project already.
• I have sorted the link alphabetically under the related section.

[Rakhi-2002]

Hello, I have added a link which is useful for C++ developers or learners. Please accept my pull request.
Thanks.

[Rakhi-2002]

Added new link under youtube channels.