Add packages + platforms to README, general rewordings & improvements

[smoogipoo]

• Adds a package list.
• Adds a platform support cross-table.
• Removes the blurb about this package being for internal use and to temper expectations. I register this as unnecessary and antithetical to fostering a mutually beneficial community.
• General rewordings because this project has expanded beyond SDL and now also contains bindings for SDL_image and SDL_ttf with additional targets arriving later as we/others need them.
• Adds readme to package.