Introduced external sources for the C++

CommunitySources.md

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+# Community sources
+
+## C++ World
+
+- https://en.cppreference.com/w/
+- https://www.cplusplus.com/reference/
+- https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines
+
+## Popular conferences
+
+- C++ Russia: https://cppconf.ru/
+- Cpp Con: https://cppcon.org/
+- Meeting C++: https://meetingcpp.com/
+- Cpp Now: https://cppnow.org/
+
+## Conference youtube channels
+
+- [C++ Now](https://www.youtube.com/user/BoostCon)
+- [C++ Russia](https://www.youtube.com/channel/UCJ9v015sPgEi0jJXe_zanjA)
+- [Meeting cpp](https://www.youtube.com/user/MeetingCPP)
+- [Cpp Con](https://www.youtube.com/user/CppCon)