doc(README.md): Boost support

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(build): -D STATIC=ON option for building a static Lean executable

On Linux, -D STATIC=ON does not work if MULTI_THREAD support is enabled.
If we search for "pthread static crash" we find other projects with the same problem.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>