Add a _Node Support Policy_ section to the project's `README.md` file
to indicate what this project, and its core team, are promising in terms
of Node runtime support.
Establishing a support policy provides reassurance to the community that
they can expect a level of functionality from `semantic-release`, while
providing guidance to the core maintainers, and everyone else that
contributes, the level of support they should espire to.
Closes#485
