Doc: document the usage scenario with build->install->find_package()

[abdes]

Add a scenario to the README file documenting how the project can help people use crypto++ via the following steps:

• clone cryptopp-cmake
• build
• install
• in the consuming project

find_package(cryptopp REQUIRED)

# compile and link a test program using crypto++
add_executable(myapp main.cpp)
target_link_libraries(myapp cryptopp::cryptopp)

Documentation should include all the commands used at each step.