Write working example for "conan install from cmake" section

[jgsogo]

Googling around I've come across this stackoverflow question. It is true, it is not working because the proposed package Hello/0.1@memsharded/testing does not exist in remotes (conan-center).

We should change documentation at https://docs.conan.io/en/latest/howtos/cmake_launch.html to use an existing package (also change README in https://github.com/conan-io/cmake-conan), and maybe add a minimum main.cpp file too.

For example, we can use the same example from the Getting started section:

• Poco/1.9.0@pocoproject/stable

• main.cpp

   #include "Poco/MD5Engine.h"
   #include "Poco/DigestStream.h"
  
   #include <iostream>
  
  
   int main(int argc, char** argv)
   {
       Poco::MD5Engine md5;
       Poco::DigestOutputStream ds(md5);
       ds << "abcdefghijklmnopqrstuvwxyz";
       ds.close();
       std::cout << Poco::DigestEngine::digestToHex(md5.digest()) << std::endl;
       return 0;
   }