Provide instructions for dynamic libraries

[alandefreitas]

The contributor guide should provide instructions for dynamic libraries.

The information about what libraries should support are here

https://www.boost.org/doc/libs/master/libs/config/doc/html/boost_config/boost_macro_reference.html#boost_config.boost_macro_reference.macros_for_libraries_with_separate_source_code.macros_controlling_shared_library_symbol_visibility

and here

https://www.boost.org/doc/libs/master/libs/config/doc/html/boost_config/boost_macro_reference.html#boost_config.boost_macro_reference.macros_for_libraries_with_separate_source_code.automatic_library_selection

One important thing to note is this needs to be properly tested (gcc, msvc, mingw, clang-cl, ...) or the probability that a blind untested implementation is not going to work is close to 100%.